MacBook Air 2012 Firmware Password Lock, what do I do?

I got a MacBook Air a while ago from an eBay seller a long time ago, and recently it showed up with a firmware lock popup when I tried to install MacOS Catalina on it because it was running a really old version of MacOS that it couldn’t update from. I talked to Apple about it and they said I could bring it in with a receipt or some type of proof of purchase of the MacBook. I brought in the eBay receipt but they said it was not enough, and I needed the actual Apple receipt that they provided, which, of course, I did not have. We did ask the eBay seller if they had the receipt but they said they did not because it was a very old MacBook. I do not have enough for another, and need help unlocking the one I currently have. Could someone provide me something to help me unlock this MacBook?