If the Apple Check Coverage page says, "We’re sorry, but we can't check coverage for this device," it could mean: The serial number was entered incorrectly. The device is a clone or counterfeit.
Beyond just being a "tech geek" hobby, checking a serial number on Apple’s Check Coverage page is your best defense in the second-hand market: Decode The Meaning Behind Your Apple Serial Number apple serial number check apple
What are you checking? (iPhone, Mac, AirPods?) Are you buying it used or new ? If the Apple Check Coverage page says, "We’re