C# Gravity Gun. Error CS0120

So I managed to get hold of a script in javaScript for making an gravity gun similar to that in half life however the game I'm working on is purely C# so I tried my hand at converting the code over to C#. I've managed to get most of the code error free but I'm still picking 1 error with no idea as to fix it. I know it has something to do with the GameObject.tag which I'm assuming is in those beginning if statements but have no idea I could fix it.

The error in question that I'm getting is Error CS0120 in which it states the following:

" Assets/Scripts/PickUp.cs(22,30): error CS0120: An object reference is required to access non-static member `UnityEngine.GameObject.tag' "

The code for it below. I have also included another script which works in conjunction with this (Again I converted the code from javaScript to C#).

PickUp Script

 using UnityEngine;
 using System.Collections;
 
 public class PickUp : MonoBehaviour {
 
     public Quaternion objectRot;
 
     public GameObject pickObj;
 
     public bool canpick = true;
     public bool picking = false;
     public bool guipick = false;
     public Vector3 objectPos;
     public GameObject pickref;
     public GameObject mainCamera;
 
       // Use this for initialization
     void Start () {
 
         mainCamera = GameObject.FindWithTag("MainCamera");
 
         pickref = GameObject.tag("pickupref");
         pickObj = pickref;
     
     }
     
     // Update is called once per frame
     void Update () 
     {
         Ray ray = mainCamera.camera.ScreenPointToRay(Input.mousePosition);
         RaycastHit hit;
 
         if (Physics.Raycast(ray, out hit, 10) && hit.collider.gameObject.tag == "pickup")
         {
             guipick = true;
         }
 
         if (Physics.Raycast(ray, out hit) && hit.collider.gameObject.tag != "pickup")
         {
             guipick = false;
         }
 
         objectPos = transform.position;
 
         objectRot = transform.rotation;
 
         if(Input.GetMouseButtonDown(0) && canpick)
         {
             picking = true;
 
             Ray rayTwo = mainCamera.camera.ScreenPointToRay(Input.mousePosition);
 
             RaycastHit hitTwo;
 
             if (Physics.Raycast( rayTwo, out hitTwo, 10) && hitTwo.collider.gameObject.tag == "pickup")
             {
                 pickObj = hitTwo.collider.gameObject;
                 hitTwo.rigidbody.useGravity = false;
                 hitTwo.rigidbody.isKinematic = true;
                 hitTwo.collider.isTrigger = true;
                 hitTwo.transform.parent = gameObject.transform;
                 hitTwo.transform.position = objectPos;
                 hitTwo.transform.rotation = objectRot;
             }
 
         }
 
         if (Input.GetMouseButtonUp(0) && picking)
         {
             picking = false;
 
             canpick = false;
         }
 
         if (Input.GetMouseButtonDown(0) && !canpick && pickObj.GetComponent<PickedUpObject>().refuseThrow != true)
         {
             canpick = true;
             pickObj.rigidbody.useGravity = true;
             pickObj.rigidbody.isKinematic = false;
             pickObj.transform.parent = null;
             pickObj.collider.isTrigger = false;
             pickObj.rigidbody.AddForce(transform.forward * 8000);
             pickObj = pickref;
         }
 
         if (Input.GetMouseButtonDown(1) && !canpick && pickObj.GetComponent<PickedUpObject>().refuseThrow != true)
         {
             canpick = true;
             pickObj.rigidbody.useGravity = true;
             pickObj.rigidbody.isKinematic = false;
             pickObj.transform.parent = null;
             pickObj.collider.isTrigger = false;
             pickObj = pickref;
         }
     }
 

PickedUpObject Code

 public class PickedUpObject : MonoBehaviour {
 
     public bool refuseThrow = false;
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 
     void OnTriggerEnter(Collider other)
     {
         if (other.gameObject.tag != "player" && other.GameObject.tag != "pickto")
         {
             refuseThrow = true;
         }
     }
 
     void OneTriggerExit(Collider other)
     {
         if (other.gameObject.tag != "player" && other.GameObject.tag != "pickto")
         {
             refuseThrow = false;
         }
     }
 }