AirPod Pro AppleCare

Hi All, I purchased a pair of AirPod Pros at Target a couple days ago. Target doesn't offer in store purchases of AppleCare while you're buying the product like other stores do (BestBuy, etc.) but I was told I could purchase the AppleCare afterwards. I have been entering the serial number into the Apple website to register them but I keep getting error messages-that the serial number isn't valid, and it once showed that the serial number is for a different device(??). I've checked and re-checked that I've entered it correctly and tried it on two different days on different devices and no luck. Any advice/insight?