why iso 15.7.3 shows error on my iPhone 6s

Two reasons

1. You need to be charged to at least 50% to perform the update. Better yet, charge up and leave the power brick plugged in while updating.

2. You need to turn off Low Power Mode to perform the update.

Low Power Mode is turned on.

Use Low Power Mode to save battery life on your iPhone or iPad - Apple Support

If you are experiencing problems updating, try deleting the iOS Update file and download it again.

First, delete the iOS Update file. Go to: 

Settings → General → iPhone Storage → Find the iOS update on the list and delete it.

After that, Restart your iPhone. Then connect to Wi-Fi, and charger, and go to:

Settings → General → Software Update and try again.

Leave your iPhone in the software update menu and preferably don't use it while downloading.

You can find more useful information here:

If your iPhone or iPad won't update - Apple Support

What is the error message?