Presumably you are being prompted for your AppleID credentials - so as to satisfy an Activation Lock that is intended to prevent reuse of the device by anyone other than the legitimate owner.
To do so you will require the AppleID an associated password that was last used to successfully activate the device - which, for the bona-fide owner, should be straightforward.
If you have acquired a previously used iPad, if the required AppleID credentials are not your own, then your will require the assistance of the former owner - as without their assistance the iPad is (and will remain) useless to you.
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/