Apple ID linked to non existent email

Tested out my cousins iPhone 7 for a few days to see if i wanted to buy it, signed into my apple account like normal. I've since given it back after factory resetting it and removing it from my apple account but now it says that an apple account, that is trying to use a gmail that does not exist, Needs to be reactivated because of security reasons. This becomes a problem when i can only do that by stating a birthday i dont know for security questions. The other option is to send an email to the email that doesnt exist to reset my password and reactivate. I have never seen this apple ID before nor the email but it somehow uses one of my passwords to login with.