There are a few common reasons why you fail to withdraw your casino funds to a MasterCard:
Casinos require players to withdraw their winnings via the same payment method they used for depositing. If you have deposited via Skrill, you cannot withdraw to a MasterCard.
The other reason for a problematic MasterCard withdrawal is related to your place of residence. There are some countries where online casino transactions to/ from MasterCard are not processed.