Diagnosis
The desk's lower limit switch needs recalibration. This happens over time as the mechanical components settle.
How to Fix It
Perform a full reset. Unplug the desk. Hold the down arrow button on the control panel. While holding it, plug the desk back in. Keep holding the button for 10 seconds after it powers on. The desk should now travel through its full range. Test the lowest height.
If That Didn't Work
Check the feet. The adjustable leveling glides might have been screwed up too high, physically preventing the legs from retracting fully. Lower them.
Heads Up
If the desk still refuses to go low after a reset, do not force it. There may be a mechanical bind inside the column. Forcing it can shear the drive screw.