Buyer claims code doesn't work

I have all the proof that the code is legit. Apparently the code really isn’t working. I need help to solve this ASAP please.

Just follow the dispute process. Upload your proof (in message between you and buyer) to show the code working. Gameflip staff will see all messages between you and buyer and will check your proof, saving you time for back and forth with the ticket.

Thanks! I thought I had to wait until they contacted me.

They would contact you to ask for proof unless you supply them to them and the buyer via the (private) messaging system.