You description appears to suggest that, you are now using a different AppleID account to that used when the iPad was last successfully “activated”. If so, this is the source of your issue; you will need to recover the original AppleID credentials in order to satisfy the Activation Lock.
An AppleID account is intended to be used for life. If you ever need to change the primary email address that identifies your account, or your secondary (recovery) email addresses or trusted telephone numbers, you simply need to update your existing AppleID account as necessary.
Change your Apple ID - Apple Support
Change your Apple ID password - Apple Support
What to do after you change your Apple ID or password - Apple Support
Change, add, or remove Apple ID payment methods - Apple Support
Few of us will have the luxury of forever keeping the same email addresses or telephone numbers - sometimes these needing to be changed for a variety of reasons. Updating our AppleID account allows us to maintain access to our data and purchases; it is easy and relatively straightforward to update the AppleID account details - accommodating life’s many changes as they occur. Switching to using an entirely different AppleID account will, unless Activation Locks are properly disabled prior to switching accounts, lead to difficulties such as an Activation Lock that cannot be resolved, leading to loss of use of the associated device.
If you do wish to change and use an entirely different AppleID, first ensure that you have disabled the Activation Lock that will otherwise cause difficulties in the future. You can disable the Activation Lock by disabling the Find My iPad service from iPad settings before trying to use a different AppleID account:
Settings > [Your Name / AppleID] > Find My > Find My iPad - set to OFF
Returning to your immediate issue, here are the Apple support pages that outline the Activation Lock - and the only mechanisms by which it can be turned off:
Activation Lock for iPhone, iPad, and iPod touch - Apple Support
Turn off Activation Lock – Apple Support
As you will discover, unless you are the owner of the AppleID that was last used to activate the device, you’ll not be able to defeat or bypass the Activation Lock. With documentary evidence that you are the original owner (such as sales receipt showing the serial number), Apple may be prepared to assist - otherwise they and nobody here will be able to help you.
If you have the necessary documentation, Apple have introduced a portal though which you may be able to request release of an Activation Lock:
How to remove Activation Lock - Apple Support & start an Activation Lock support request.
However, recovery of the AppleID credentials should be possible…
If you have forgotten your AppleID or associated password, these can be recovered here:
https://iforgot.apple.com/
To recover your credentials, you’ll need access to any one of the following:
- Your primary email address mailbox that corresponds with your AppleID
- Any of the secondary/recovery email addresses that should be configured for your AppleID account
- Any of the trusted telephone numbers (fixed line or Cellular/Mobile) that are associated with your AppleID
Unless you AppleID account has not been fully/correctly configured, or has been seriously neglected, recovery of the AppleID and password should not be difficult.
More information about recovery of your AppleID:
If you forgot your Apple ID - Apple Support
More information about recovery of your AppleID password:
If you forgot your Apple ID password - Apple Support
When you recover access to your credentials, you would be well advised to log-in to your AppleID account from a web browser - and verify/update any email addresses and trusted telephone numbers:
https://appleid.apple.com/