There are several scenarios that could lead your bank to refuse the payment:
Your bank card does not authorise or restricts internet payments.
Your purchase and withdrawal limits imposed by your bank have been reached.
Your bank identifies this payment as risky or potentially fraudulent.
Troubleshooting
To resolve this problem, proceed as follows:
Check in your bank's customer area that online payments are not restricted, that your balance is sufficient and that your limits have not been reached.
Contact your bank, explaining that you are trying to process a payment with Bitstack, and ask the bank to authorise all future payments.
Try processing the payment again manually in the application by clicking on the Activities tab, selecting the failed transaction and clicking on the Retry button. It may be that the block is only temporary.
Remove your card as a payment method and add it again. This will authorise the use of the card with Bitstack with your bank.
Add another card as a payment method.