While reports of users unable to download apps from the App Store in the wild are few, the common thread seems to be that the problem is contained to users of macOS Mojave 10.14.
When attempting to purchase or download apps from the App Store, users are getting an error that reads, “We could not complete your purchase,” followed by the detail text “cancelled.” The error has also been reported when a user attempts to update an app. So far, only users who are currently running macOS Mojave 10.14. have reported the issue. There have been no reports of the issue for those running macOS Catalina 10.15.
AppleInsider was first notified of the problem by a tipster pointing to the Apple support forums,. Sporadic mentions of the issue can be found on social media as well.
Read more at AppleInsider.com