Q: Incorrect Apple ID - Cannot be used to unlock this iPad

That means you aren't entering the id that the iPad is locked to i.e. the account that you were using in iCloud on it.

Have you tried : If you forgot your Apple ID - Apple Support ?

Or, and if you were the original purchaser of the iPad (i.e. it wasn’t bought used), 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. original receipt with the serial number on it