If your App Store is broken....
... as has happened for most of us Mojave users here, it is imperative to stop loading up the forums, a place where Apple employees who can act on this seldom visit.
Instead, use this feedback link that a responsible person at Apple WILL see:
Like a politician's mailbag, enough reports on an issue have proved to get action. Don't get emotional, just post facts. If this happened after you installed Security Update 2021-002, include that.
Going on and on in the forums won't get this fixed! There are a couple of workabouts posted here but relying on those doesn't get the issue fixed properly either.
Make sure Apple hears your keystrokes by using the feedback link.
