Diagnosis
Your opener is signaling a safety sensor obstruction. The door path is blocked or the alignment is off.
How to Fix It
Go to the garage door opening. Check that nothing is in the path. Look at the two small sensor units on either side of the door track near the floor. The green LED on the receiving sensor must be lit solid. If it's off or blinking, the beam is broken. Realign the sensor brackets so they face each other directly. Tighten the wing nuts. The green light will come on solid when aligned.
If That Didn't Work
If the green light stays off after alignment, the wiring is damaged. Check the thin 2-wire cables running from each sensor back to the motor unit. Look for pinches, cuts, or loose connections at the terminal block on the back of the opener.
Heads Up
Never bypass the sensors by holding down the wall button. The door can close with enough force to injure someone or crush an object.