Accessing variable from another object

I am trying to access the ThirdPersonCharacter script (specifically the isPoweredUp variable) that is attached to my mainPlayer object, from the power-up.

The goal is when the player collides with the power up, for it to flag the isPoweredUp, and then destroy the power up object.

Here is what I have done so far:

using UnityEngine;
using System.Collections;

public class pickUp: MonoBehaviour
{  

    public GameObject mainPlayer;
    public GameObject pickupPrize;
    public GameObject playerScript;

    private ThirdPersonCharacter thirdPersonCharacter;

    void Awake() {
        mainPlayer = GameObject.FindGameObjectWithTag("Player");
    }

    void Start()
    {

        pickupPrize = GameObject.FindGameObjectWithTag("Pickup");
        thirdPersonCharacter = mainPlayer.GetComponent<ThirdPersonCharacter>();

    }

    void  OnTriggerStay(Collider thePlayer){

        {
            thirdPersonCharacter.isPoweredUp = true;
            Destroy(pickupPrize);


        }
    }
}

On my mainPlayer object, I have a public variable called isPoweredUp, currently set to false.

When i try to run this I get the following error:

The type or namespace name ThirdPersonCharacter could not be found

What am I doing wrong here?

EDIT:

This is the ThirdPersonCharacter script attatched to my mainPlayer GameObject:

Using UnityEngine;

namespace UnityStandardAssets.Characters.ThirdPerson
{
    [RequireComponent(typeof(Rigidbody))]
    [RequireComponent(typeof(CapsuleCollider))]
    [RequireComponent(typeof(Animator))]

    public class ThirdPersonCharacter : MonoBehaviour
    {
        [SerializeField] float m_MovingTurnSpeed = 360;
        [SerializeField] float m_StationaryTurnSpeed = 180;
        [SerializeField] float m_JumpPower = 12f;
        [Range(1f, 4f)][SerializeField] float m_GravityMultiplier = 2f;
        [SerializeField] float m_RunCycleLegOffset = 0.2f; 6
        [SerializeField] float m_MoveSpeedMultiplier = 1f;
        [SerializeField] float m_AnimSpeedMultiplier = 1f;
        [SerializeField] float m_GroundCheckDistance = 0.1f;

        Rigidbody m_Rigidbody;
        Animator m_Animator;
        bool m_IsGrounded;
        float m_OrigGroundCheckDistance;
        const float k_Half = 0.5f;
        float m_TurnAmount;
        float m_ForwardAmount;
        Vector3 m_GroundNormal;
        float m_CapsuleHeight;
        Vector3 m_CapsuleCenter;
        CapsuleCollider m_Capsule;
        bool m_Crouching;
        public bool isPoweredUp;
        ...
    }
}