What is your client version?
Does this issue persist all the time? Or what changes do you do before this issue happened?
Does anyone else have the same issue?
I tested in my lab with Teams version 1.3.00.30866(64-bit). When I start a Teams meeting with a guest, the sharing function can work well as below.
If I start a screen sharing from the chat window, it only contains desktop and Windows.
It recommends you make sure your client is the latest version and clear the client cache. Then have a try.
If still no luck, log in Teams web client(https://teams.microosft.com) and try again.
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