This occurs because one or more of the Generation criteria (Application type, Transaction type, Payment method and/or Subtype) already exists in another mapping.

For example, when creating a mapping for a pledge payment, you enter applied to: Pledge, with payment method: credit card, credit card subtype: MasterCard and receive the error.  This means there is already a transaction mapping for pledge payments paid by MasterCard credit cards.

To locate the already existing transaction mapping, click on each row in the list of mappings, click Edit, and review the generation criteria details (such as subtypes).