Hi I get this error error CS0131: the left-hand side of an assignment must be a variable, property or indexer how to fix ?

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class WeaponScript : MonoBehaviour {

    public GameObject Weapon;
    public GameObject Postion;
    public AudioSource KnifeGrab;
    public bool KnifePickedUp;

    void Update()
    {

            if (Input.GetKeyDown("f")) 
            {
             KnifePickedUp = false;
             Postion.SetActive (false);
            }

             if (Input.GetKeyDown("f")) 
            {
             Weapon.SetActive (true);
             KnifePickedUp = true;
             KnifeGrab.Play();
            }

            if (Input.GetKeyDown("2")) 
            {
             Weapon.SetActive (true);
             KnifePickedUp = true;
             KnifeGrab.Play();
            }

            if (Input.GetKeyDown("1")) 
            {
             Weapon.SetActive (false);
             KnifePickedUp = false;
            }
    }

   public void playSoundEffect()
   {
if (Input.GetKeyDown("f") && KnifePickedUp = true)
       {
        KnifeGrab.Play();
       }
    }
}