My iPhone 11 pro can’t update to iOS15

Make sure you:

• Are using a WiFi connection (not a hotspot).

• Have your battery charged at least 50%. Better yet, keep it plugged in during the update.

• Are not using a VPN connection.

• Do not have power save mode turned on.

• Are not running iOS 14.7 Beta.

Do you see any other error other than unable to update? You are not providing enough information to fix right now. Are you connected to a stable wi-fi, or a computer with the latest version of iTunes? Do you have ample space on the iPhone for the update? You will need around 4GB I believe. You can see this for help, or provide more info. Update your iPhone, iPad, or iPod touch - Apple Support

You can also look at the suggestions in this User Tip Updating iOS: common issues and solutions - Apple Community