Hi ronaldo sanders. Welcome to the forum. Suspect a defective ignition resistor going to the ignition coil or control box. It drops voltage to a safe level during driving, but during cranking, battery voltage will already be low, the resistor is bypassed to maintain a strong spark. When the resistor is open, there will be no spark so the engine won't run, but since it is bypassed during cranking, it does run as long as you hold the switch in the " run" position.
Chrysler had the same problem in the late '70s. It was so common, a lot of people carried a spare $3.00 resistor with them. In their case, the resistor was bypassed by a terminal on the ignition switch so you could shift to " drive" while holding the ignition switch and the engine would run. In gear, the neutral safety switch would turn the starter off. GM and Ford used a tap on the starter solenoid to bypass their resistors so you were stuck on the side of the road.