The first thing to check is how free are the wheels to turn- get the drive wheels off the ground, put a socket on 1 of the lug nuts & try turning the wheels- they shouldn't be that hard to turn by hand.

If it fails that test, disconnect the brake line from the master cylinder on the right side (as seated on the machine) & try the test again. If the wheels turn more easily then either the master cylinder is faulty or the brake actuation rod needs adjusting.

If the wheels turn easily with the first test then you may have a sticking inching valve or a worn trans. Either one of these issues will allow the engine to rev to governed speed as if the trans selector is in neutral. To test these- have lift in open area, block the wheels, rev engine to governed speed- keep engine reved & place trans selector in f or r. At a minimum the engine should bog down under the stress of the engaged trans. If there's really no difference in engine rpm then a trans issue is present. At that point another set of checks are required.