We are fellow user, we can't remove the iPad from the account. (I've asked the hosts to remove the serial number from your post.)
You are typing in the id and password of the account that you were using in Settings > iCloud on the iPad, it shows on that account if you log into it on http://www.icloud.com ? If the iPad is on iOS 9 : If you can't activate your iPhone, iPad, or iPod touch after installing iOS 9.3 - Apple Support
You could try going to an Apple Store (make a Genius Bar appointment if you do) or try contacting iTunes Support online (https://www.apple.com/emea/support/itunes/contact.html) and see if they can remove the iPad from the account - they are likely to want to see proof of ownership before they might help e.g. your receipt with the serial number on it