App Store update issue? "We could not complete your purchase. cancelled"
This is a known issue. These steps were suggested here:
https://robservatory.com/fix-a-could-not-complete-your-purchase-app-store-error/

Quit the App Store app.

Switch my DNS to another provider.

In Terminal, paste this command and press Return:
open &quot;$TMPDIR/../C/com.apple.appstore/&quot;

Confirm that the Finder opened a window to the com.apple.appstore folder, then drag everything there to the trash.

Back in Terminal, paste this line and press Return:
killall -9 appstoreagent

Relaunch the App Store app.


I have had this work, and I've had this not work. I've found that logging out and logging back in to the App Store has helped, and then there are times when it hasn't.
Nothing has solved it long-term, all of these are temporary fixes so far.
It just started recently, so I assume something changed on Apple's end, but I have no idea if/when a permanent solution may be forthcoming.
If anyone has a better technique for dealing with this, I hope they will share it.
Another Non-Solution Option
If you go into the Mac App Store and check for pending updates, you will see the apps that need to be updated.
If all else fails, you can delete the app and then download the new version from the Mac App Store. That works, although it is obviously not ideal.