Security light flashes engine will not start

The flashing security light means the system does not recognize the key you are using. Try a different key that has worked in that truck before. If neither key works then it is likely that there is a problem in the ignition switch, body control module or the wiring between them. To narrow down which item it is you need to use a dealer level scan tool to watch which components are being activated when you turn the key on. Then once the faulty part is replaced you will need to re-program the system to recognize the keys again.