This error message when show when the payment method used for the transaction is not mapped within your general ledger. You would need to find the application that the payment is applied to and add that payment method to that mapping.
  1. Click Administration > General Ledger Setup
  2. Click Define Transaction Mappings
  3. Choose the category that the transaction falls under (For example, sales orders would fall under the ticketing category while back office transactions would fall under development)
  4. Click on the payments tab > click the arrow next to the revenue type that your transaction falls under
  5. Click edit > click the arrow next to the "with payment methods section" add the payment method
  6. Click save
Alternatively you can also add a brand new mapping to the transaction mappings that includes the revenue type and payment method of the unmapped transaction
  1. Click Administration > General Ledger Setup
  2. Click Define Transaction Mappings
  3. Choose the category that the transaction falls under (For example, sales orders would fall under the ticketing category while back office transactions would fall under development)
  4. Click on the payments tab > Click add
  5. Add the Debit and Credit account information
  6. Click the arrows to choose your revenue type and payment method type
  7. Click save