The default definition of the default device can be found in /usr/share/alsa/pcm/default.conf. If it does not redirect to a driver-specific default, it is defined like this:
The plug plugin implements automatic sample rate/format conversion.
Most drivers do have their own ...
I had this issue after not installing third party software and update in the first installation step in Ubuntu 20.04 LTS.
You should just shift from gdm3 to lightdm to resolve this issue by opening the terminal and typing:
sudo apt-get install lightdm
sudo dpkg-reconfigure lightdm
After entering these commands if you are asked that only one of gdm3 or ...