How to update iPhone 7 from ios11 to ios 13
Phone will not update
If your iPhone, iPad or iPod touch won’t update - Apple Support
User Tip updating iOS
Updating iOS: common issues and solutions - Apple Community
You can only update to iOS 14.3.
but it still unable to update..
If you are having problems updating, then see if this helps: https://support.apple.com/en-us/HT201263