Enable to update I phone

See If you see an error when you update or restore your iPhone, iPad, or iPod https://support.apple.com/en-us/HT201210 and

iOS update and restore errors https://support.apple.com/en-us/HT204770

Get help with over-the-air iOS updates          

Have you tried using iTunes on a computer or Finder if running Catalina to do the update?

 Update software on iOS devices in iTunes on Mac