Why my iphone 6s plus everytime i update there is an error occurred?
Why I can’t update the new iOS 15
Assuming you're trying to update your device wirelessly (via wifi), see this suppport article:--> If your iPhone, iPad or iPod touch won’t update
If the issue persists, try to update it via computer:
Learn how to update your device manually if you're using a Mac with macOS Catalina or later, or if you're using a Mac with macOS Mojave or earlier or a Windows PC.
If you still get no help, contact Apple Support directly, using this link:--> Get Support
Regards
Giulio