I tried to install a root-encypted Debian 11 (Bullseye, netinst, daily build 2021-03-29, cryptsetup) next to the existing Win 10 as Dual boot on a strict UEFI-only system (Dell Inspiron 17 7706, BIOS 1.3.0, 16GB RAM, 512 GB NVMe).
Eventually I could shrink the Win10-partition, UEFI-boot the Netinst-Image, connect the Wifi (AX201) and install the encrypted Linux system, but
efibootmgr fail with
"EFI variables are not supported on this system"
modprobe efivars resp.
modprobe efivarsfs fail either with
"modprobe: FATAL: Module efivarsfs not found in directory /lib/modules/5.10.0-5-amd64/"
Copying efivarsfs.ko there would make modprobe to work, but not grub-install/efibootmgr, leaving the system unbootable with Debian Linux (Win 10 boots).
I tried all the hints from "EFI variables are not supported on this system" but no success. Also tried the solutions from https://wiki.debian.org/GrubEFIReinstall no success either. Also tried rEFInd but it would not see my unenrypted boot-Partition at all.
/sys/firmware/efi/efivarsfs/ exists, but is empty. Secure boot is off.
Any ideas left?
Next step would be to try a plain new install of Ubuntu 20.10, but I'd like to stay with debian.
Thanks in advance for any help.