Why I can’t update software?

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 

Here is more information on How to  ➔ Update your iPhone, iPad, or iPod touch - Apple Support

Axel F.