We could not complete your purchase – Fix
I guess you landed here because your Mac displayed the message We could not complete your purchase while trying to update your Mac. Most commonly the root cause is due to an authentication issue. Granted, Apple’s messaging could be a bit more specific, but here is a possible solution for fixing the update problem.
In a browser (Safari, Chrome …) head to Apple Account page. Click on Apple ID and password and login. Chances are that you’ll get some sort of warning about your device. Carefully confirm that it is you wanting to login. If you did not get any warnings at all, this probably won’t fix the issue and you should further research possible connectivity issues.
If you encountered problems with your password, make sure to reset it. After successful authentication, try to update your Mac again and see if this solved your issue.