Can't reference an imported script in one of my own scripts or vice versa.

I have two scripts that I want to reference to each other. One is a script that I have made that controls the core of the game, while the other (the imported one) is the firstpersoncontroller script. All I want to do is set the lock cursor boolean to true or false, given if certain variables are certain values in the core script, but I can't seem to reference the scripts to each other. I have been referencing scripts successfully before, but now Unity just can't seem to understand the type of the scripts I'm trying to reference. Is there something I should know?

Edit:

Sure thing! I can only show so much, so I'll try to compress these scripts a bit to show everything important.

Firstpersonscript:

 using System;
 using UnityEngine;
 using UnityStandardAssets.CrossPlatformInput;
 using UnityStandardAssets.Utility;
 using Random = UnityEngine.Random;
 
 namespace UnityStandardAssets.Characters.FirstPerson
 {
     [RequireComponent(typeof (CharacterController))]
     [RequireComponent(typeof (AudioSource))]
     public class FirstPersonController : MonoBehaviour
     {
         [SerializeField] private bool m_IsWalking;
         [SerializeField] private float m_WalkSpeed;
         [SerializeField] private float m_RunSpeed;
         [SerializeField] [Range(0f, 1f)] private float m_RunstepLenghten;
         public float exhaustTime = 5;
         public bool isExhausted = false;
         public bool initiateCooldown = false;
         public float cooldownTime = 7;
         public float delayTime = 2;
         [SerializeField] private float m_JumpSpeed;
         [SerializeField] private float m_StickToGroundForce;
         [SerializeField] private float m_GravityMultiplier;
         [SerializeField] private MouseLook m_MouseLook;
         [SerializeField] private bool m_UseFovKick;
         [SerializeField] private FOVKick m_FovKick = new FOVKick();
         [SerializeField] private bool m_UseHeadBob;
         [SerializeField] private CurveControlledBob m_HeadBob = new CurveControlledBob();
         [SerializeField] private LerpControlledBob m_JumpBob = new LerpControlledBob();
         [SerializeField] private float m_StepInterval;
         [SerializeField] private AudioClip[] m_FootstepSounds;    // an array of footstep sounds that will be randomly selected from.
         [SerializeField] private AudioClip m_JumpSound;           // the sound played when character leaves the ground.
         [SerializeField] private AudioClip m_LandSound;           // the sound played when character touches back on ground.
 
         private Camera m_Camera;
         private bool m_Jump;
         private float m_YRotation;
         private Vector2 m_Input;
         private Vector3 m_MoveDir = Vector3.zero;
         private CharacterController m_CharacterController;
         private CollisionFlags m_CollisionFlags;
         private bool m_PreviouslyGrounded;
         private Vector3 m_OriginalCameraPosition;
         private float m_StepCycle;
         private float m_NextStep;
         private bool m_Jumping;
         private AudioSource m_AudioSource;

Core script:

 using UnityEngine;
 using System.Collections;
 using UnityEngine.SceneManagement;
 using UnityEngine.UI;
 
 public class PlayerActivate : MonoBehaviour

I'm only showing you a little from the core script, because I feel the problem is originating from the first person script, as the core script hasn't given me problems in the past until I needed to reference this new script.