The banner message is fairly self-explanatory - in that you have a network connectivity issue.
A recommended first step is to restart your WiFi Router. To restart, disconnect power from the Router for at least 30 seconds - then reconnect power to restart. After the Router finishes its reboot cycle, which may take many minutes, force-restart your iPad:
- On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
- On an iPad with Face ID: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.
After restarting the Router and iPad, you’ll be able to re-attempt the software update.