How can I resolve this error I'm getting in my movement script?

For my player controlling script, I have attempted to do it so the player can only jump two times, I've not had any errors in the script itself but when I save it and go back on Unity, the error I keep getting is this:

error CS0120: An object reference is required for the non-static field, method, or property Collision.gameObject

This is my code:

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

public class playerController : MonoBehaviour
{
    public float speed;
    public float power;
    private Rigidbody2D rb;
    int numberofJumps = 0;
    public int doubleJump = 2;

    void Start()
    {
        rb = GetComponent<Rigidbody2D>();
        numberofJumps = doubleJump;
    }

    void Update()
    {
        if (Input.GetKey(KeyCode.D))
        {
            rb.AddForce(Vector3.right * speed);
        }

        if (Input.GetKey(KeyCode.A))
        {
            rb.AddForce(Vector3.left * speed);
        }

        if (Input.GetKeyDown(KeyCode.W))
        {
            if (numberofJumps > 0)
            {
                Jump();
            }
        }
    }

    public void Jump()
    {
        rb.AddForce(Vector3.up * power, ForceMode2D.Impulse);
        numberofJumps -= 1;
    }

    void OnCollisionEnter2D(Collision2D col)
    {
        if (Collision.gameObject.tag == "ground") 
        {
            numberofJumps = doubleJump;
        }

    }
}

The error seems to be specifically regarding this line:

if (Collision.gameObject.tag == "ground")