I was having the same problem these past two weeks...I reinstalled Microsoft Autoupdate, I reinstalled all my Windows apps, still the same error message as soon as I tried to update: "Updates are temporarily unavailable. Please try again later".
Then....I remembered I had used the CCleaner app startup tool and a similar one in Clean My Mac to disable several startup items I thought, in my infinite wisdom, didn't need to be firing up from the get go. Silly me...
As I reviewed with both apps tonight the list of what was Disabled vs Active, I discover several items I had turned off a couple of weeks ago that had 'microsoft updater...' i them. I turned them all back on and voila! Auto update is working again.
I'd really like to apologize for saying nasty things about Microsoft and its products the past two weeks..None of this was there problem...I will do penance by denying myself chocolate for a week.. That should equal the score. I'm using Big Sur, latest update on a 2021 iMac 27.