The reasons for this situation are as follows:

1. The credit card has not activated the online payment function: Some credit cards require the online payment function to be activated in the bank or Alipay before they can be used on Alipay.

2. Insufficient credit card limit: If the credit card limit is insufficient to cover the current order amount, Alipay will prompt that the credit card payment cannot be used.

3. Alipay account abnormality: If there is an abnormality in the Alipay account, it may result in the inability to use credit card for payment. You can try to log in to your Alipay account again or contact Alipay customer service to solve the problem.

4. Merchants do not support credit card payment: Some merchants may not support credit card payment and only accept other payment methods.