Diagnosis
The down force setting is too low. The opener thinks it's hitting an obstruction because it doesn't detect enough motor strain to close the door against normal spring tension.
How to Fix It
Adjust the down force. On the back of the motor unit, find the two adjustment dials labeled 'UP FORCE' and 'DOWN FORCE'. Turn the DOWN FORCE dial clockwise one notch. Test the door closure. Repeat only if necessary. Stop once it closes reliably.
If That Didn't Work
The door's spring tension may be incorrectly set. A door that is too heavy for the opener will cause this. Disconnect the opener and manually lift the door halfway. It should stay in place. If it falls, the springs need adjustment by a professional.
Heads Up
Increasing down force too much defeats the safety reversal system. After adjustment, test the safety reversal by placing a 2x4 on the floor under the door. The door must reverse immediately upon contact.