This issue has mostly affected iPad 2 models with SIM cards for Cellular data service.
If you have a cellular iPad 2 model, try removing the SIM card from the SIM card tray, located on one of the long sides of the iPad ( you may need something small or pointy or the original SIM card tray tool that came with your Cellular service iPad 2 to open the tray ), remove the SIM card, try either a simple restart of your iPad 2 AND/OR try a hard reset of your iPad by holding down both the Home and sleep/wake buttons simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons.
Then, try re-activating your iPad 2 without the SIM card in the tray.
If successful, return the SIM card back into the tray.
If no joy,
Leave the SIM card installed, first, and try to re-activate your iPad 2 with it connected to a computer running iTunes.
If STILL no joy,
Try taking out the SIM card and try the re-activation with your iPad 2 connected to the computer and iTunes, once again.
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.
Good Luck to You!
