My iPad shows ‘puk exhausted’ when I charge it up

when i was younger, my grandpa gave me this ipad pro max (i think) first model, and it always said ‘puk exhausted’ whenever it charged up. he left it logged into his icloud. he was a guy that forgot his passwords easily. I dont know any passwords to the icloud or anything, so i use someone else’s sim code so it connects my device to theirs, and i know them. I cant go and ask a electronics store because i would have to tell the person, how to fix?

[Re-Titled by Moderator]