Diagnosis
Dust or spider webs on the photo-eye lenses break the infrared beam. The opener sees this as an obstruction.
How to Fix It
Use a clean, dry microfiber cloth. Wipe the plastic lens on each sensor. Do not use cleaner or water; it can leave a film. Check alignment. The LED on the receiving eye (green lens) must be solid, not blinking.
If That Didn't Work
Sunlight can overpower the sensors in late afternoon. Shade the receiving eye with your hand to test. If it works, you need to install the sun shields that came in the box or adjust the mounting angle.
Heads Up
Never splice or extend the sensor wires. They are 24VAC and a specific gauge. Longer wires cause voltage drop and permanent faults.