I'm not able to update my iPhone 13's iOS

You need both a WiFi and cellular connection to perform the update, it looks like you have no cellular signal. If you don't have access t ocellular at your location, you can 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

Click on this link on what to do if you can't update your phone -

If your iPhone or iPad won't update – Apple Support (UK)

Your phone does not have a cellular connection. The update cannot run until you have a working connection to your carrier....and.....a Wi-Fi connection.