If all required question are answered this occurs when a disposition = "Transfer to Another EMS Agency", and the agency selected does not have a NEMSIS code set.
Go to Customization-->Lists-->EMS Agencies and enter appropriate code. Check to see that codes have been entered for all Agencies.
Generally you will set the NEMSIS code to one of the following:
4221015 Other EMS Responder (air)
4221017 Other EMS Responder (ground)