When you say the lever is on tight do you mean that the lever is very hard to pull as if that is the case and the wheel is moving towards the brake pad that moves then it could be that you just need to loosen the cable off as essentially the brake may be permanently activated if that cable is too tight.

The disc itself won't be adjustable but you must have adjustment points on the calipers as calipers are designed to go on numerous different bikes that may all have slightly different spacings so need to have a degree of adjustability to make them work properly. I'd suggest watching a quick you tube video on adjusting cable disc brakes as that may give you some insight on the parts you have and how you can adjust them.

Good luck!