App Store on Mojave: "We could not complete your purchase. cancelled"

Solution 1:

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 "$TMPDIR/../C/com.apple.appstore/"
• 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 "$TMPDIR/../C/com.apple.appstore/"*
killall -9 appstoreagent 

Solution 2:

I fixed the problem by opening iTunes and signing in there. As soon as I signed into iTunes with the same account as the App Store, the problem went away.