so little code, so many errors,
1) you can not define properties inside a function:
void Start()
{
public Transform missionCheck;
public float missionDistance = 1f;
public LayerMask missionMask;
public int missionNext = 0;
}
2) variables must be defined before used:
missionStep =
3) object property used without allocating object (missionCheck.position)
missionStep = Physics.CheckSphere(missionCheck.position, missionDistance, missionMask);
4) case statements in a switch must have a break