I have a Troy Bilt self propelled 21 inch mower.
The power drive does not work right. It does not pull
Good at all.When the wheels are raised off the ground
They turn good. Also when pulling the mower backward
The wheels are locked.

I assume front wheel variable speed drive. Post a model #.

You do need to disengage the drive when pulling the mower backwards.

First, inspect the tread of the wheels. If, worn, forget it. Replace the wheels. If MTD says they are not available, check your mower shop. Aftermarket wheels may be.

If I's like the one I'm familiar with, take off the plastic piece on top and toward the front of the mower. This will expose the drive mechanism.

There seems to be a few failure modes:
1) belt
2) friction disk or just adjustment
3) bearings on the driven shaft

Inspect and repair.

Check for a stretched belt, and check to see that the cable is moving the engagement lever the full motion.