A quick fix for Ubuntu users whose Zoom screen sharing only shows “Whiteboard” in the basic sharing options, caused by Wayland being the default display server.
Your issue is probably due to the fact that Ubuntu 21.04 release use Wayland as its default display server instead of Xorg.
To verify that what composition manager you use type:
echo $XDG_SESSION_TYPE
if it’s echo back:
wayland
It’s mean your ubuntu uses Wayland.
Switch to Xorg
Go to your login screen and try login with Xorg:

Now try sharing in zoom. If that’s working for you - you can change to Xorg permanently using:
edit the /etc/gdm3/custom.conf file:
sudo gedit /etc/gdm3/custom.conf
Next un-comment # WaylandEnable=false by removing the # at the beginning:
WaylandEnable=false
Save your changes and run this command to force Xorg:
sudo gedit /etc/environment
When file opens, add line _QT_QPA_PLATFORM=xcb and save it.
