Tighten the lock nut. This nut usually takes a 9/16” wrench.
NOTE: Do not over tighten. Since this is a lock nut, just tighten it until it is gently snug. It needs to be free to pivot. If you have a torque wrench, tighten to a torque of 7--8 ft-lb (8--11 N•m) then loosen the lock nut 1/8 turn.