Diagnosis
The door is out of balance or the track is binding. The opener's motor is detecting excessive resistance.
How to Fix It
Disconnect the opener. Pull the red emergency release cord to manually operate the door. Lift the door halfway and let go. It should stay in place. If it slams down, the torsion springs are weak and need professional service. Also, manually move the door up and down the full path. It should move smoothly with minimal effort. Listen for grinding and look for bends in the metal track.
If That Didn't Work
The opener's internal force settings are calibrated too low. Adjust the UP and DOWN force limit knobs on the side of the motor unit. Turn the appropriate knob clockwise 1/8 turn. Test. Increase only as much as necessary for smooth operation.
Heads Up
Increasing the force settings on a badly balanced door is dangerous. It allows the door to close with enough power to crush something. Fix the balance first.