iOS 15

See below, the computer method should succeed if the Wi-Fi OTA method does not.

Check 

• you are on a good Wi-Fi, 
• on charge with more than 50% charged, 
• no VPN installed 
• low power mode turned off. 
• Fully working cellular network connection via your SIM.
• Check Settings, General to make sure you have no Beta profile installed.

Resources:-

Update phone

Update your iPhone, iPad, or iPod touch - Apple Support

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