If you’re using Ubuntu and your Zoom client have only “Whiteboard” on it’s basic sharing options. You need to follow those instructions:

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:

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echo $XDG_SESSION_TYPE

if it’s echo back:

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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 

file:

sudo gedit /etc/gdm3/custom.conf

Next un-comment

by removing the # at the beginning:

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WaylandEnable=false

Save your changes and run this command to force Xorg:

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sudo gedit /etc/environment

When file opens, add line “QT_QPA_PLATFORM=xcb” and save it.