First check portal.office.com to make sure subscriptions are valid. If they are its corrupt files causing it. Here is the fix:
- Launch each Office app installed on your Mac, click on the application name (eg Word) menu and choose Sign Out. Then quit the application.
- Open "Finder", the location of all of the files
- Hold the command key (ALT on normal Keybaord) and select "GO" on the top ribbon. Select "Library"
- Click on the Group Containers folder
- Locate the following three files/folders and drag them to the Trash
UBF8T346G9.ms
UBF8T346G9.Office
UBF8T346G9.OfficeOsfWebHost - Launch an Office application and sign in to check if you can now edit documents.
-Might need to update Microsoft apps if you cannot sign in.
-When signing in, you might also need to "active" each app (bottom left)