Reverse thrust on RC Jet

[GMastrob9]

I have a Motion RC F4 with a SpeedyBee F405 Wing. Using Crossfire and RadioMaster. All controls work, testing has been done, etc.. However, I then tried to add the reverse thrust. I set up the motor controller for reverse thrust. I put it on output 10 on my radio. The receiver in INAV shows output 10 working. I plugged the reverse thrust wire into it output 10 on my SpeedyBee and all still seems okay. I then added in the mixer tab a new mixer rule with Servo 10 in the mixer and RC Channel 10. When I do that I get a PCM INIT error. Not sure how to get this reverse thrust working. Thanks!