There are several reasons why the payment method that you choose to get a welcome bonus may be rejected. One of the most common reasons is because your details were wrong. You should double-check them if you get an error message.
If you use credit or debit cards, a possible reason for why your payment method was rejected may be because of your bank. Several banking institutions block their customers from using their cards to gamble. In this case, your activity was probably red-flagged and blocked. You can avoid this issue by changing your payment method.