Many many with the same issue, myself included.
I was on the phone with MS Support for 5 hours (not a typo) to no avail so out of frustration I ended up buying Office 2024 for $180 + tax.
I should have tried this first. It should work for you:
Uninstall all MS apps from Mac
Download/Remove the MS License (used the Microsoft_Office_License_Removal_2.7.pkg from the MS website),
Download Office 2021 again from:
learn dot microsoft dot com / en-us / officeupdates / update - history - office - for - mac
>>> Use the 16.113 (26100421) release, NOT the latest 10/22/26 version which causes the problem
Install and activate it.
Disable the autoupdate feature so it does not update again (if it does update you will have the problem again).
Then disable the annoying Autoupdate banner with this:
Rename the auto update app by prefixing it with a “_”
'/Library/Application Support/Microsoft/MAU2.0/_Microsoft AutoUpdate'