I've been so fustrated with trying to get my old iCloud account off my old iPhone

I forgot the password and recovery key to my iCloud account. I've called Apple and sent a receipt for proof of purchase, but they said it's invalid, because the SN doesn't match which doesn't make sense. Is there another way to just erase everything off the phone myself?

Go to iCloud.com when it asks for your password, select forgot password and answer your security questions and you can reset your password.

If you don't know the answers to your security questions then you're out of luck. They're very specific questions where you don't just forget the answers, if you don't know it then you lied in your answer and now you're dealing with the consequences