Just like any other transaction, you should contact your merchant acquirer, the card issuer, or the card network.
Coin is a device that stores a user’s existing payment cards and allows users to use those stored cards at stores and other merchants. Coin is not a credit card issuer, like Chase or Bank of America, or a credit card network, like Visa or MasterCard. When Coin is swiped through the POS terminal for a transaction, it will transmit the selected card’s payment information just like the original plastic card. The issuer and card network then process the transaction the same way that they do for the plastic version of the card.