I got exchange from Best buy

I don't speak so much English so this is the problema
On Sunday I went to the best buy and made a purchase of a gift card then by mistake the code ended in the afternoon of that same day I went to see what could be done about the gift card and yesterday the manager gave me some sheets that According to the real code, a new code, rather, so the code didn't work right now I came and they fixed the problem, so they changed my card but the code doesn't work, I want to know what the error is here