How to update my iPhone 7
How to update my iPhone 7+
iPhone 7 Plus, iOS 14
It tells you right in your screen shot, you need a WiFi connection to perform the update.
You also need to charge your battery over 50% to do the update. Better yet, charge it and leave the power supply plugged in while updating.
You can also update using a computer.
Click on this link for instructions on how to update your phone using a computer
Update iOS on iPhone - Apple Support