Make sure the clutch pedal is up all the way. Sometimes they like to stick a bit near the top of travel and be down just enough for the clutch switch to be triggered. After that you would need to find someone with a laptop and Cat ET software that can monitor the switch inputs to see what the ECM is seeing as to why the cruise/brakes are being inhibited.

Check the brake lite switch there is usually one for cruise and jakes or could be one also check speed sensor on output shaft of trans check for a loose yoke nut and also check the senser for the proper ohm also the wires are commonly rubbed through

Thanks. I will check.