I am running Mojave and have found that on 2 Macs the App Store gets into an endless loop of warning messages that it is unable to do the updates. I have had to force quit the app.
Strangely it just worked when I tried it just then so, somehow, the problem seems to be resolved.
Don’t be surprised if it comes back. I’ve seen the same for three days now without finding a consistent solution. Once it stopped after switching to my Store account, but that didn’t work the next two times. The second time I went one app page and clicking on the download cloud fixed it for all the other apps. The last time, today, I had to delete the app and re-install it.