What cause 'NO_RECEIVER_CASE_BE' in Synchronous interface

Dear Experts,

What cause error 'NO_RECEIVER_CASE_BE' in a synchronous interface?

Checking at the Receiver Determination of the Interface, there's only 1 receiver and no condition is put for that receiver.

I thought that way, the receiver should always be only one and I wonder what cause the error to be 'NO_RECEIVER_CASE_BE'.

Thank you,

Suwandi C.