Original answer, preferred: In order to be able to uncheck the folders in "choose folders", I needed to go to onedrive settings -> backup -> manage backup, then was able to stop backing up Desktop, Documents, and Pictures. Once I stopped that, I was able to uncheck the folders in "choose folders". This was highly non-obvious.
EDIT August 2022: I ran in this issue again and had a completely different solution that also worked: in choose folder (though the original answer above is still preferred because it's a more direct method that directly addresses the root cause). You can't uncheck the Documents folder itself due to "can't stop syncing", but if you EXPAND the folder you'll see a checkbox titled "Files in Documents" or something like that. I was able to uncheck THAT, and that also automatically unchecked Documents. I repeated for the other folders and it worked fine.
