Hello . The cruise control switches in the steering wheel may be disconnected and causing this concern, I have also seen where some drink was spilled on the switch and caused poor contacts. Both of these switches are connected together and the signal is sent by one wire through the clockspring. So if on/off work - then the clockspring is ok and a switch could be bad.
The other possible concern could be caused by a brake light switch that has failed or is out of adjustment. These switches can only be adjusted in one direction, thats inward.So you may try holding up on the brake pedal while trying the cruise control. If the cruise then works, - replace the brake switch .
The rest of the system can be tested using the dealer scan tool. There are sensor inputs to the engine computer that could cause the system to not work.
I hope this info is helpful. Please ask if you have any questions , Thanks Scott.