Why do I get an error when downloading/updating from the App Store?

Unfortunately I don’t have any control over the app download/update process. The App Store is operated by Apple and the downloaded/updated apps are stored on Apple servers.

Apple has an official help article for these issues: https://support.apple.com/en-us/102331

If you end up contacting the Apple Support, they’ll most likely tell you to contact the third-party app developer (me) first; if they do, you can link to this page and emphasize that since the download/update is managed by the App Store and the App Store is an Apple app, only Apple can help.