On a failed payment, validation, or refund what does this "Reason for Failure: ..." error message mean?

The message immediately following "Reason for Failure: " is typically a message directly from the gateway or even the credit card issuer. The messaging of the error will likely describe parts of the problem with the payment method. 

Some examples are:

• Insufficient Funds
• General Decline
• Declined
• AVS Error, Address does not match. 
• The refund amount attempted is more than the amount remaining of the same payment
• "Exceeds daily limit" or similar language indicates it is more than your gateway allows. 
• Refund for a payment that is over 60 days old and the gateway has rules in place
• Some other gateway specific rule. 

There will be customer service steps to take around this outside of Fusebill. Have your customer double check their payment method information including the name on the card, security information, and the address of the payment method itself. 

They may also need to check with their card issuer to see if there is something that could be potentially blocking them from purchasing things online or getting recurring bills. 

You may also need a member of your team to reach out to your gateway in order to get more information on the response.