error CS0103: The name `Cardboard' does not exist in the current context

I'm attempting to make a cardboard game, and I get this error: DogWalkScore.cs(59,69): error CS0103: The name `Cardboard' does not exist in the current context I've scoured the site for a solution, but nothing has worked, I'm still doing something wrong. Thank you in advance, your help is greatly appreciated. The script code is below.

 using System;
 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 
 public class DogWalkScore : MonoBehaviour
 {
     private const int RIGHT_ANGLE = 90;
     private int score;
 
     public Text scoreText; 
 
     // This variable determinates if the player will move or not 
     private bool isWalking = false;
 
     GvrHead head = null;
 
     //This is the variable for the player speed
     [Tooltip("With this speed the player will move.")]
     public float speed;
 
     [Tooltip("Activate this checkbox if the player shall move when the Cardboard trigger is pulled.")]
     public bool walkWhenTriggered;
 
     [Tooltip("Activate this checkbox if the player shall move when he looks below the threshold.")]
     public bool walkWhenLookDown;
 
     [Tooltip("This has to be an angle from 0° to 90°")]
     public double thresholdAngle;
 
     [Tooltip("Activate this Checkbox if you want to freeze the y-coordiante for the player. " +
              "For example in the case of you have no collider attached to your CardboardMain-GameObject" +
              "and you want to stay in a fixed level.")]
     public bool freezeYPosition;
 
     [Tooltip("This is the fixed y-coordinate.")]
     public float yOffset;
 
     void Start()
     {
         head = Camera.main.GetComponent<StereoController>().Head;
         score = 0;
         SetCountText();
     }
 
     void Update()
     {
         // Walk when the Cardboard Trigger is used 
         if (walkWhenTriggered && !walkWhenLookDown && !isWalking && Cardboard.SDK.Triggered)
         {
             isWalking = true;
         }
         else if (walkWhenTriggered && !walkWhenLookDown && isWalking && Cardboard.SDK.Triggered)
         {
             isWalking = false;
         }
 
         // Walk when player looks below the threshold angle 
         if (walkWhenLookDown && !walkWhenTriggered && !isWalking &&
             head.transform.eulerAngles.x >= thresholdAngle &&
             head.transform.eulerAngles.x <= RIGHT_ANGLE)
         {
             isWalking = true;
         }
         else if (walkWhenLookDown && !walkWhenTriggered && isWalking &&
                  (head.transform.eulerAngles.x <= thresholdAngle ||
                  head.transform.eulerAngles.x >= RIGHT_ANGLE))
         {
             isWalking = false;
         }
 
         // Walk when the Cardboard trigger is used and the player looks down below the threshold angle
         if (walkWhenLookDown && walkWhenTriggered && !isWalking &&
             head.transform.eulerAngles.x >= thresholdAngle &&
             Cardboard.SDK.Triggered &&
             head.transform.eulerAngles.x <= RIGHT_ANGLE)
         {
             isWalking = true;
         }
         else if (walkWhenLookDown && walkWhenTriggered && isWalking &&
                  head.transform.eulerAngles.x >= thresholdAngle &&
                  (Cardboard.SDK.Triggered ||
                  head.transform.eulerAngles.x >= RIGHT_ANGLE))
         {
             isWalking = false;
         }
 
         if (isWalking)
         {
             Vector3 direction = new Vector3(head.transform.forward.x, 0, head.transform.forward.z).normalized * speed * Time.deltaTime;
             Quaternion rotation = Quaternion.Euler(new Vector3(0, -transform.rotation.eulerAngles.y, 0));
             transform.Translate(rotation * direction);
         }
 
         if (freezeYPosition)
         {
             transform.position = new Vector3(transform.position.x, yOffset, transform.position.z);
         }
     }
 
     private void OnTriggerEnter(Collider Rat)
     {
         if (Rat.gameObject.CompareTag("Pickup"))
         {
             Rat.gameObject.SetActive(false);
             score = score + 1;
             SetCountText();
         }
     }
     void SetCountText()
     {
         scoreText.text = "Score: " + score.ToString();
     }
 }