If more than two channels are assigned to the AN/PRC-77 all channels >2 have an undefined frequency.

I think the problem is caused here where the AN/PRC-77 appears to be assigned two channels:
The AN/PRC-77 does not have any channels, but a frequncy range of 30.00 MHz to 75.95 MHz and a frequency spacing of 0.05 MHz means the AN/PRC-77 can transmit on 919 discrete frequencies.