Diagnosis
The down-limit setting is too low. The door is hitting the floor before the motor stops, so the safety reversal activates.
How to Fix It
Find the travel limit adjustment screws on the side of the motor unit. One is for UP (usually black), one for DOWN (usually white or green). With the door closed, press the wall button to open it fully. Now press the button to close it. As it closes, watch it. When the door just touches the floor, immediately press the button again to stop it. Turn the DOWN limit screw counterclockwise 1/4 turn. Press the button to open it, then close it again. Repeat until it closes and stays down without reversing.
If That Didn't Work
The door springs are weak. The opener is working too hard to close it, triggering the force sensor. Test the door manually. It should balance halfway open. If it's heavy, the springs need adjustment by a professional.
Heads Up
After adjusting limits, test the safety reversal. Place a 2x4 on the floor under the door. Close the door. It must reverse immediately upon touching the wood. If it doesn't, the force settings are too high and unsafe.