This error occurs when a record type associated with the Pledge or the Pledge Installment is Inactive.  The most frequent culprit in this situation is the Pledge Installment record type.

Check to see if there are any inactive record types that could be impacting the creation of the Pledge Installment:
  1. Navigate to Setup > Customize > Opportunity > Record Type.
  2. Check the list for any Inactive record types.
  3. If any necessary record types (such as Pledge Installment) are not Active, click Edit next to the appropriate record type.
  4. Mark the checkbox for Active and save.