iOS 15

Restart your iPhone. Restart your WiFi Router. Then try again.

Sill no luck, try another WiFi Network or update via computer.

Update your iPhone, iPad, or iPod touch

https://support.apple.com/en-ie/HT204204

The truth is that we do not know which error has occurred either. We will therefore have to tackle this by generalizing things. When updating from iOS 14 to iOS 15, Make sure that your iPhone is setup to meet the following requirements before you attempt to update the iOS version-

• Your iPhone must have a fast Wi-Fi connection; you cannot update over cellular data or a cellular hotspot
• The iPhone can not be in Low Power mode
• The battery state of charge must be above 50%
• Your iPhone must have at least 6GB or more free storage space
• Your iPhone cannot be connected through VPN
• You should leave the phone plugged in until it finishes updating, both to assure that it remains above 50% and to keep Wi-Fi enabled (Wi-Fi turns off when the phone sleeps if not connected to power).

Axel F.