I purchased a MacBook Pro month back. I know all Mac products come with one year warranty. But whenever I try to check my warranty coverage and enter my serial number and purchase date I get this message 'We’re sorry, but we are unable to complete

Is he an authorized reseller? And, what year/model is it?

In any case, you should contact Apple by phone or chat and explain the situation. The reseller may have had it for a while and the warranty has expired (Apple starts the warranty when the item is shipped).

Unless the seller is an authorized seller, the sale will not automatically be logged at Apple, so Apple does not know if and when the device was bought.You need to make sure you have a purchase receipt; then call Apple and tell them that you are unable to register your product. They may want to see the receipt.

Where did you buy it?

And, was it new in a factory sealed box or a store display or used?