When you mount the wheels the bike needs to be on its "feet", both wheels on the ground. Assuming vertical dropouts, open and close the skewer making sure the axle is fully seated in the dropouts with enough tension on the skewer. If you have horizontal dropouts there could be another problem.

Any tuning, testing, or adjusting needs to be done with the bike upright, wheels down. You need a way to turn the pedals with the wheels off the ground.