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I installed kubernetes on Ubuntu server 20.04 .
Master configured with flannel network without any problem .
Initialize command:

kubeadm init --pod-network-cidr=10.10.10.0/24 --apiserver-advertise-address=172.16.200.10

Network interfaces on master:

cni0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1450                                                                                                                                                                            [4/651]
        inet 10.10.10.1  netmask 255.255.255.0  broadcast 10.10.10.255
        inet6 fe80::8e1:9ff:fe5e:56fb  prefixlen 64  scopeid 0x20<link>
        ether be:cc:5a:4b:f0:c0  txqueuelen 1000  (Ethernet)      
        RX packets 6565  bytes 400726 (400.7 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 3121  bytes 291909 (291.9 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

docker0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        inet 172.17.0.1  netmask 255.255.0.0  broadcast 172.17.255.255
        ether 02:42:9e:d3:e7:73  txqueuelen 0  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 172.16.200.10  netmask 255.255.255.0  broadcast 172.16.200.255
        inet6 fe80::20c:29ff:fe6b:dbf8  prefixlen 64  scopeid 0x20<link>
        ether 00:0c:29:6b:db:f8  txqueuelen 1000  (Ethernet)
        RX packets 7134  bytes 713894 (713.8 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 9077  bytes 2726670 (2.7 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

flannel.1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1450
        inet 10.10.10.0  netmask 255.255.255.255  broadcast 10.10.10.0
        inet6 fe80::ec9f:dfff:fe19:b2bd  prefixlen 64  scopeid 0x20<link>
        ether ee:9f:df:19:b2:bd  txqueuelen 0  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 14 overruns 0  carrier 0  collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 222414  bytes 41222254 (41.2 MB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 222414  bytes 41222254 (41.2 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

veth92c2b55e: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1450
        inet6 fe80::bccc:5aff:fe4b:f0c0  prefixlen 64  scopeid 0x20<link>
        ether be:cc:5a:4b:f0:c0  txqueuelen 0  (Ethernet)
        RX packets 3252  bytes 244283 (244.2 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 1602  bytes 148781 (148.7 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

vethdc37182b: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1450
        inet6 fe80::d84e:c0ff:feda:9875  prefixlen 64  scopeid 0x20<link>
        ether da:4e:c0:da:98:75  txqueuelen 0  (Ethernet)
        RX packets 3313  bytes 248353 (248.3 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 1540  bytes 144798 (144.7 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

but nodes result this error on /var/log/syslog after join to cluster.

Jan 27 08:08:33 k8s-node1 kubelet[5393]: E0127 08:08:33.534423    5393 pod_workers.go:191] Error syncing pod 90b895af-b046-4149-ac18-bbec859c37a2 ("kube-flannel-ds-vsgx8_kube-system(90b895af-b046-4149-ac18-bbec859c37a2)"), skipping: failed to "StartContainer" for "kube-flannel" with CrashLoopBackOff: "back-off 2m40s restarting failed container=kube-flannel pod=kube-flannel-ds-vsgx8_kube-system(90b895af-b046-4149-ac18-bbec859c37a2)"

This is nodes status:

root@k8s-master:~# kubectl get node -o wide
NAME         STATUS   ROLES                  AGE   VERSION   INTERNAL-IP     EXTERNAL-IP   OS-IMAGE             KERNEL-VERSION     CONTAINER-RUNTIME
k8s-master   Ready    control-plane,master   42m   v1.20.2   172.16.200.10   <none>        Ubuntu 20.04.1 LTS   5.4.0-62-generic   docker://19.3.8
k8s-node1    Ready    <none>                 35m   v1.20.2   172.16.200.20   <none>        Ubuntu 20.04.1 LTS   5.4.0-62-generic   docker://19.3.8

What is this error ?
How can fix that ?

Update
This is exists containers in Node1:

root@k8s-node1:~# docker ps -a
CONTAINER ID        IMAGE                  COMMAND                  CREATED             STATUS                      PORTS               NAMES
358f8cdc6141        f03a23d55e57           "/opt/bin/flanneld -…"   15 seconds ago      Exited (1) 12 seconds ago                       k8s_kube-flannel_kube-flannel-ds-vsgx8_kube-system_90b895af-b046-4149-ac18-bbec859c37a2_7
e3a40bcc9db3        f03a23d55e57           "cp -f /etc/kube-fla…"   11 minutes ago      Exited (0) 11 minutes ago                       k8s_install-cni_kube-flannel-ds-vsgx8_kube-system_90b895af-b046-4149-ac18-bbec859c37a2_0
7ccb8514bed7        43154ddb57a8           "/usr/local/bin/kube…"   11 minutes ago      Up 11 minutes                                   k8s_kube-proxy_kube-proxy-mvbw2_kube-system_5090f36a-4178-4318-aa15-19c61239045f_0
27ae4b2a1157        k8s.gcr.io/pause:3.2   "/pause"                 11 minutes ago      Up 11 minutes                                   k8s_POD_kube-flannel-ds-vsgx8_kube-system_90b895af-b046-4149-ac18-bbec859c37a2_0
507f2d4ab72c        k8s.gcr.io/pause:3.2   "/pause"                 11 minutes ago      Up 11 minutes                                   k8s_POD_kube-proxy-mvbw2_kube-system_5090f36a-4178-4318-aa15-19c61239045f_0

Container Error:

root@k8s-node1:~# docker logs k8s_kube-flannel_kube-flannel-ds-vsgx8_kube-system_90b895af-b046-4149-ac18-bbec859c37a2_7
ERROR: logging before flag.Parse: I0127 08:16:06.400204       1 main.go:519] Determining IP address of default interface
ERROR: logging before flag.Parse: I0127 08:16:06.402530       1 main.go:532] Using interface with name eth0 and address 172.16.200.20
ERROR: logging before flag.Parse: I0127 08:16:06.402721       1 main.go:549] Defaulting external address to interface address (172.16.200.20)
W0127 08:16:06.402840       1 client_config.go:608] Neither --kubeconfig nor --master was specified.  Using the inClusterConfig.  This might not work.
ERROR: logging before flag.Parse: I0127 08:16:07.100353       1 kube.go:116] Waiting 10m0s for node controller to sync
ERROR: logging before flag.Parse: I0127 08:16:07.100618       1 kube.go:299] Starting kube subnet manager
ERROR: logging before flag.Parse: I0127 08:16:08.101341       1 kube.go:123] Node controller sync successful
ERROR: logging before flag.Parse: I0127 08:16:08.101408       1 main.go:253] Created subnet manager: Kubernetes Subnet Manager - k8s-node1
ERROR: logging before flag.Parse: I0127 08:16:08.101423       1 main.go:256] Installing signal handlers
ERROR: logging before flag.Parse: I0127 08:16:08.102395       1 main.go:391] Found network config - Backend type: vxlan
ERROR: logging before flag.Parse: I0127 08:16:08.103389       1 vxlan.go:123] VXLAN config: VNI=1 Port=0 GBP=false Learning=false DirectRouting=false
ERROR: logging before flag.Parse: E0127 08:16:08.104363       1 main.go:292] Error registering network: failed to acquire lease: node "k8s-node1" pod cidr not assigned
ERROR: logging before flag.Parse: I0127 08:16:08.105069       1 main.go:371] Stopping shutdownHandler...

Why in nodes, flannel containers exited ?!!!

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That --pod-network-cidr 10.10.10.0/24 sounds wrong.

If you do a kubectl describe node <your-master-node-name>, I would bet there is something like a PodCIDR: 10.10.10.0/24. Your first node is already eating up the whole pod network CIDR, as you planned one that is too small.

Once flannel first starts on a Kubernetes node, it would allocate itself with a /24 subnet, taken in the pod network range. I would assume you're just out of IPs, for your second node to join the SDN.

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