Spektrum has always been a problem with Castle ESC's. I got this from the Castle website.....
Why does my ESC not beep and show a solid red when trying to calibrate with a Spektrum SR6100AT receiver?
We have found that Spektrum has added a safety feature that interferes with our normal calibration and manual programming process. Our ESCs require you to power up the ESC at full throttle; the Spektrum AR6100AT receiver will not boot up properly until the throttle trigger is at neutral.
The solution we have found is to power up the ESC at neutral, wait for the receiver LED to turn solid amber, then apply full throttle; this allows the receiver to boot up properly.
WARNING: REMOVE Pinion gear or disengage motor from spur gear! You must apply full throttle BEFORE the ESC arms (double chime after LiPo cell count), but AFTER the receiver boots up and goes solid yellow. If the ESC arms and then you apply throttle the motor will run! We have found that waiting until the Auto LiPo feature counts the 3rd cell the receiver is operational and full throttle can be applied