You should check this article: If you can't activate your iPad - Apple Support. If you can't activate your iPhone - Apple Support
If your iPad is a cellular model, some have reported that temporarily removing the sim allows activation to complete.
Otherwise, 5 possible causes for that error message:
1] Activation server is actually down. Very rare occurrence, and of short duration when it
does happen. Solution - try again.
2] Your firewall and/or antivirus software are preventing the necessary connection. Solution -
temporarily disable your firewall and/or antivirus software until the activation completes.
3] computer being used for activation has been used to jailbreak an iOS device in the past.
Solution - use a different computer.
4] iPad has been jailbroken. Your favorite search engine may provide some direction, but jailbreaks often prevent
activations/updates from succeeding. Per Terms of Use for this forum, discussion of jailbroken iPads is
prohibited.
5] your region and/or timezone settings are inconsistent with your actual location. Solution - set region and/or timezone correctly.
There has been discussion in this forum that Apple has changed the way the verification process works
and the old version of Safari on your iPad 2 does not pass the security checks to reach the verifying
server.