My cruise control will not engage, the lights.

My cruise control will not engage, the lights comes on, but its blinking and the truck is not engaged to cruise. Now when I first start the truck up, before I put it into any gear I can press the cruise to on and the light want blink and I can use the cruise, but if I get up to a speed of more than 55mph it goes back to blinking and once again I cant utilize cruise anymore