Restore iPad and install the lastest version of iPadOS and attempt activation again

Start here >>> If you can't activate your iPhone or iPad (Wi-Fi + Cellular) - Apple Support

If you are experiencing difficulties with (re)activation of your iPad, you may have more success if you connect your iPad to a PC (with iTunes installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of macOS). Once connected to your computer, your iPad will be detected - and you should be able to activate using iTunes.

1. Make sure that you have the latest version of macOS or the latest version of iTunes.
2. Check that your computer has an Internet connection.
3. With the cable that came with your iPad, connect your iPad to your computer. Locate your device on your computer.
4. Wait while your computer detects and activates your iPad:

• If you see an alert that shows "Set up as New" or "Restore from Backup", it means your iPad is activated.
• If you get an error message that says the "activation information was invalid" or "activation information could not be obtained from the device", use recovery mode to restore your iPhone.

A restart of your WiFi Router may also be helpful.

To restart you home router, disconnect power - then wait at least 30 seconds before restarting. When you router has rebooted (which may take several 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.

Should it be necessary to escalate your issue to Apple Support, the quickest route to engaging with Apple Support will be via the  Get Support link at top-right of this page.

Otherwise, if you need expert help, you can obtain hands-on assistance from the technicians at your local Apple Store or Apple Authorised Service Provider (AASP):

Genius Bar Reservation and Apple Support Options - Apple

Here are some links for the actions that shemaka provided

Update your iPhone, iPad or iPod touch – Apple Support

If you can't update or restore your iPhone, iPad or iPod touch – Apple Support

If your iPhone, iPad or iPod touch won’t update - Apple Support

iOS update and restore errors – Apple Support

1. Try performing a recovery mode restore: If you can't update or restore your iPhone, iPad, or iPod touch. And if that doesn’t work try

Learn how to update your iPhone or iPad to the latest version of iOS or iPadOS.

You can update your iPhone or iPad to the latest version of iOS or iPadOS wirelessly.

If the update doesn't appear on your device, use your computer to update your device manually. Learn how to update your device manually