Does this issue persist all the time?
Are there any error messages when you click screenshare button?
In this case, we recommend you try to clear Teams cache and restart your client to see if it can be fixed.
If there is no luck, you can also try to reinstall Teams client to see if it works for you.
I did a test in my environment (Ubuntu 18.04) and it can share desktop normally, but we cannot see mouse pointer in the screen (it is a known issue). And I will also do a test in Ubuntu 20.04 to see if we can reproduce this issue and give a feedback to you.