This can occur when creating a brand new wce file from the ground up. Creating a wce file through TestKit will assume the input source of the modulated signal to be a RF modulated signal. Since the A304 generates signals in digital base band, the wce file would have to be configured to accept a dbb signal as a source.
The TestKit GUI does not offer to select the input source so that it may get written into the wce file.
Please see FAQ section on how to create a custom wce file for Data Throughput operating on an A304 or A604 platform.