First, can you move the lever? If no - you need some WD40 or equivalent.

If yes, but the sight won't twist (the lever is ONLY a friction-lock) you either need MORE WD40, or, as was done in some cases the upper sight base has been pinned so as to prevent adjustment by the troops.

This can only be ascertained by removing the sight from the barrel.

If it IS pinned, you can either "hold off" when you shoot, live with un-centered groups, or attempt to remove the pin - which route was not intended by the applicator threrof. Some damage is inevitable, but, it will all be concealed.