App Store on Mojave: "We could not complete your purchase. cancelled"
From https://ericasadun.com/2020/05/22/broken-app-store-downloads-on-mojave-we-could-not-complete-your-purchase/:

Quit App Store
In Terminal: open &quot;$TMPDIR/../C/com.apple.appstore/&quot;
In Finder: trash everything in that folder including any pending updates / stuck items.
In Terminal: killall -9 appstoreagent 
Relaunch App Store
Done

If you feel confident with Terminal you can also quit the App Store and then run
rm -r &quot;$TMPDIR/../C/com.apple.appstore/&quot;*
killall -9 appstoreagent