DIAGNOSIS The plastic internal gear on the height adjustment mechanism is stripped. This is a common wear part.
FIX You need to replace the cutting height adjustment assembly. Order the specific part kit. To install, remove the blade disc and the central cover plate on the mower's underside. The old assembly unclips, and the new one snaps in.
IF THAT DIDN'T WORK If the new knob still spins, the mating gear on the mower body's chassis may also be damaged, requiring a more involved repair.
HEADS UP Do not force the knob. Setting the height with stripped gears gives inconsistent results and damages the lawn. Replace the assembly promptly.