Page Error: "There was a problem with our checkout"

I have had two customers try to check out on my store and each one receives a message stating that I do not deliver to their address. They both live in the United States which is where my market is. The One customer said that when she cleared her cookies and her history, she was able to go on to my site and place an order. Is this going to keep happening or is there a way for me to permanently fix this so I don’t lose customers?