Diagnosis
The down-limit is set too high, or the force adjustment for closing is too low. The opener thinks it's hit an obstacle (the floor) and safety-reverses.
How to Fix It
First, try the 'force adjustment'. On the side of the motor unit, you'll find two dials, usually labeled 'Up Force' and 'Down Force'. Turn the 'Down Force' dial up by one or two increments (usually clockwise). Try closing the door again. If it now closes fully, you're done.
If That Didn't Work
You need to adjust the physical down travel limit. With the door closed, locate the limit adjustment screws on the motor unit (they're different from the force dials and are often labeled 'Up Limit' and 'Down Limit'). Turn the 'Down Limit' screw a quarter-turn in the direction that allows more travel (check for the correct direction—it's often counterclockwise). Test the door. Small adjustments are key; a quarter-turn at a time.
Heads Up
Before adjusting limits, make sure the door itself moves smoothly by hand when disconnected from the opener. A sticky or binding door will cause this problem no matter how you adjust the opener. Also, after increasing the down force, do the safety reversal test: place a 2x4 block of wood on the floor under the door. When the door closes, it must reverse immediately upon hitting the wood.