I posted something very similar back in September of 2020:
Still running Mojave 10.14.6. For no reason at all, out of the blue on Feb. 22, 2021, the Mac App Store suddeny shows “no updates” even though I have seven updates (when I select the update tab on the left to view the updates, I see a blank white screen).
When I try to download a “known” update (or any software for that matter) by typing it in manually in the Mac App Store search and click on the download button, the wheel spins and spins and…
Force quitting appstoreagent in Activity Monitor solves the problem but when I quit the Mac App Store, the problem returns.
Nothing new has been installed for months so I decided to install Mojave 10.14.6 Security Update 2021-002 today, did a safe boot, SMC reset, PRAM reset, logged in and out of my account on the Mac App Store numerous times. None of this has helped.
Is anyone else experiencing this? It seems to me that the last time this happened, it had something to do with Apple’s trust certificates on their end because the problem did eventually fix itself on its own even though I tried a dozen things to try and resolve it.