Apple id

You can't change your Apple ID to an existing Apple ID using that process. You would need to sign out of the Apple ID on the iPad by going to Settings [ name on top] > Sign Out. Then sign back in using your original Apple ID.

If you are trying to merge the 2 Apple ID's, you cannot do that either. If you wanted to keep both ID's, it may be best to set up Family Sharing to share data between each one.

Set up Family Sharing - Apple Support