If you bought AppleCare then you should have the agreement number. Did you receive that? You can contact Apple to see if they show anything but ultimately you may need to contact Costco. If Apple doesn’t show AppleCare for your device or you didn’t receive anything from Costco showing the details for your purchased AppleCare + purchase then only Costco can determine what happened. Either they actually sold you their insurance or something else is going on. I assume because you say when you try to look it up it gives you the option to purchase AppleCare that it’s been less than 60 days. You’ll want to get this resolved ASAP so your window for AppleCare won’t close if for some reason Costco didn’t give you the right thing. Good luck!