Welcome to the Apple Support Communities. From your post, I understand that you're having some trouble getting your iPad activated after erasing your settings. When you try, you get an error that the iPad could not be activated. I'd be happy to help!
Based on what you've described, I would recommend making sure you've done all the steps outlined here: If you can't activate your iPhone - Apple Support . Even though the article is specific to an iPhone, the troubleshooting steps are the same for an iPad. If your iPad is Wi-Fi and Cellular capable, you can follow all the steps in the article. If your iPad is Wi-Fi only, you can skip any steps that deal with the SIM, or contacting the carrier.
Should you still get an activation error after using the above linked article, please reach out to Apple Support directly for additional assistance. You can do that here: Contact Apple Support .
Have a great day!