I keep getting "sharing restricted on this call" message and can not see how to fix it on my Mac

if you are the Host of the meeting remember you need to Login to the Zoom app and then start the meeting to join as host. If you are not the host but were made an alternative host the same thing applies. if you simply click the invite link and you are not logged into the Zoom app then you will join the meeting but Zoom doesn't know if you are host or cohost. you can verify this by clicking the participant list and find your name. if it has (guest) next to your name you are not logged into the Zoom Client. Once logged in as host or cohost you can click the  share screen and change share settings for the meeting to restrict or allow all. or you can promote someone to cohost to be able to share.