I'm sure you have already done this, but worth double checking the preflight checkout. I've been caught out before with a cable not connected or connected in the wrong place.
Also, I found this in the field service manual, may be helpful ?
No IF Found
This error is detected by the level detector and the 85102 A21 IF counter board. This is done using an algorithm in the 85101C math CPU to calculate the IF frequency. The A21 counter and the CPU check if the IF is within range and has enough power. If not, the CPU initiates a search mode that attempts to relocate the test set VTO harmonic to re-establish the 20 MHz IF. This message is displayed if the IF is not found, or is not within range.
Probable cause of failure:
• RF source signal may be absent, low in power, or at an incorrect frequency
• 85102 A21 IF counter board
• Test set samplers. Be sure that the test set A3 summing amplifier assembly has not biased the samplers off. Measure the voltage on the sampler control pin to each sampler to verify TTL levels. 0 volts
turns the sampler on and approximately 3 volts or greater turns it off. You can press the S-parameter front panel keys to toggle the samplers.
• Defective cable in the IF signal path
• Test set component failure: A weak VTO, bad power amplifier, step recovery diode, or 4-way power splitter in the VTO
• 85101C A5 math CPU
• 85102 stop sweep cable not connected
• Occasionally, the 85102 A20 sweep ADC can cause this error, accompanied by a Source Sweep Sync error message.
Troubleshooting:
• Check the RF source to test set cable.
• Run the 85102 service program in the Run All mode.
• Run 85101C service program test 1 (CPU A5), and the 85102 tests 2 (A20) and A (A21).
• Perform the unratioed power test in “Unratioed Power Failures” to see if the samplers are working.
• Check the IF test set interconnect cable.
If the test set is an 8514A or 8515A, compare the S11 and S22 traces. If one looks bad and the other looks good, the 85102 and 85101C are probably working. Therefore, suspect a problem with the test set or the IF test set interconnect cable. If both S11 and S22 look bad, first suspect a problem with the 85102 or the source.
NOTE This check does not work for 8512A and 8513A test sets. However, you can compare the a1 and b1 channels for unratioed power levels. Refer to “Unratioed Power Failures.”