Apple ID not valid + Email dosent exist anymore

So need help. Just spent an hour with Apple tech on the phone and he couldn't help! So hoping someone has an idea.

MacBook Air 2020. Looking to reste and erase to pass on. Ask me to sign out of an Apple ID account that no longer exists. I cannot reset as the email linked to this account no longer exist (same with the telephone number - old Land line) Its possible that I deleted this account a while back on a different device. The technician could find it on his system... so just said sorry and that was that !!

So what do I do to erase and reset my Mac??