My guess is that the problem is not resolved. I'm having the same issue.
You can confirm it by typing the following command into Terminal and seeing if you have a recent manifest.plist file. If you are comfortable with this kind of "surgery," you can delete all the files in the folder, reboot, and open the App Store again.
If you go into the same folder and do not see a manifest.plist file at all, then you will be having the same problem I am.
Bottom line, I've tested this across several machines and the Mojave Security Update 2021-002 10.14.6 seems to have broken the App Store's ability to download or update apps
If anyone here has a working App Store after applying the patch that dropped a couple days ago, please post here.
Another way you can test to see if this is a problem is to click on your user name from within the App Store app. It will show all your previous purchases, however, instead of an "open" oval, everything will look like a download cloud. I believe this is because there is no manifest.plist file to tell the App Store what apps you have.
Anyway, the terminal command to open the the App Store folder is:
open $TMPDIR../C/com.apple.appstore/
I have an escalated case with Apple Engineering and will post back when I hear something.
If it's not obvious, this pretty much sucks because anyone with this problem cannot buy, re-download, or update apps until it is fixed.
If you have not yet applied the Mojave Security Update 2021-002 10.14.6, my 2cents is to wait.
