A couple different scenarios may have caused your transaction to be declined.
The merchant selected 'debit' instead of 'credit' to run the transaction.
The merchant selected 'gift' instead of 'credit' to run the transaction.
The merchant attempted to process the purchase for a greater amount than what is available on the card. If this happens, you should ask the merchant to run a “split-tender” transaction which will allow you to pay a portion of your purchase with your card and the remaining balance with another form of payment