Assuming that the drive belts are installed correctly. Start the mower, move the speed selector to its highest setting and let the clutch out with the forward/reverse lever in neutral. Now push the clutch back in and move the speed selector to a low setting, and shift the forward/reverse selector into either gear and let the clutch out. Mower should move.
I am thinking the belts are out of sync and need to be reset. A few years ago i was having to do this for customers about one a week.