"Refer to card issuer" decline message for Merchant Warrior means that the bank is not providing a specific reason and is instructing the cardholder (the guest) to contact them for more details. This can happen for various reasons, such as insufficient funds, a security block on the card, or incorrect card details.
The guest needs to contact their bank or card-issuing company to understand and resolve the issue. Once the guest confirms the issue is fixed, you can collect the payment manually.