Approve this iPad?

See if anything here helps.  If necessary, you can use account recovery as explained in the link at bottom of the article.

Verification code - how to get one and sign in with two-factor authentication.   

My iPad Is Showing Approve This iPad

Go to one of your other devices signed in to iCloud to approve this iPad

If you aren't getting a code on one of your other devices, see what I posted above.