DIAGNOSIS The logic board is throwing a fault code. Alternating red/green flash is the opener's "alert" mode.
FIX Press and hold the black button on the logic board for 6 seconds until the LED turns solid green. Release. The opener will re-learn its force limits. Run the door through a full open/close cycle using the wall console. This often clears the fault.
IF THAT DIDN'T WORK A sensor wire is likely shorted or disconnected. Check the two safety sensor wires where they connect to the logic board (terminals 1 and 2). Look for a break or pinch, especially at the door header bracket.
HEADS UP Never bypass the safety sensors to test. The door will not operate without a solid green light on both sensor eyes.