I just updated to Catalina yesterday.  (Ignoring the fact that it took a long time AND hung on the “setting up” step.)  This morning I opened up Microsoft Teams, hopped on a meeting, went to share my screen and got a pop up.  In general (and this morning), my instinct is to dismiss a pop up until I can actually read it later.

However, this meant that I now could not share my screen on my Teams meeting.  My only option for sharing was my screen background.  Not very useful.  Once I ended the meeting, I couldn’t find or remember what the pop up said.

It’s a new setting, where you have to allow “screen recording” to Teams.

Go to your mac’s Settings.  Then Security & Privacy.  Then Screen Recording.  Select Teams and you’re good to go.