How do I resolve expired coverage for a recently purchased MacBook Air?

Update your proof of purchase - Apple Support

To my knowledge you must contact Apple by phone. You must also provide a proof of purchase from the Apex apple reseller in Mauritius, so be sure to have it in hand before you call Apple.

No Apple employees participate in these user-supported, public communities.

Was it made clear to you at the time of purchase that you were purchasing a previously owned MacBook Air, or was it sold to you as though it were a new device? If the former, its warranty period (12 mo) was already about to expire.

Ask APEX if they can hook you up with AppleCare on that MacBook Air, or failing that, ask for a full refund.