Possible solution WITHOUT a SIM card (WiFi ONLY iPad ).
Connect your iPad to a computer running the latest iTunes.
Force your (still plugged-in) iPad to restart by pressing and holding the power (Sleep/Wake) and Home buttons at the same time. Keep holding them down until you see the Recovery mode screen. You probably need to keep holding these down for longer than you think: the screen will go black then after a few seconds, and the Apple logo will appear, but you need to keep holding the buttons for another 10 seconds or so before the iPad goes into Recovery mode.
You'll be given the option to restore or update in iTunes. Select Update.
iTunes will now try and attempt to reinstall iOS 9.3.5.
If the update process takes more than 15 minutes, your iPad will exit recovery mode, and you'll need to repeat the above step.
Some other users here reported seeing the error message again during this process, but says you should ignore it and wait.
After the update is finished, leave your iPad connected to the computer. You'll now need to complete the activation setup process for your iPad using iTunes.
IF still no joy at this point, leave your iPad connected and force another restart of your iPad as I described earlier in this reply post.
This time your iPad should ask if you want to restore it.
Once the restore is finished, you will be asked to select whether you want to activate the iPad as a "new" device or if you want to restore it from your last backup. Selecting the new iPad option will format your iPad so it is like new and restoring it will retrieve your data again.
If still no joy,
Contact Apple support via the contact support link located at the upper right hand corner of every Webpage of these Apple support communities.
Use the Device Performance and Connectivity support categories.
OR
by installing and using the Apple Support app found in the iOS App Store.
Tap the iOS App Store link below.
https://apps.apple.com/us/app/apple-support/id1130498044
OR
Call contact Apple via the support numbers found in the link bekow.
Contact - How to Contact Us - Apple
Tap the link below.
https://www.apple.com/contact/
Best of Luck to You!