The best overloaded method match for `UnityEngine.Rigidbody.AddForce(UnityEngine.Vector3, UnityEngine.ForceMode)' has some invalid arguments

I have this error.
can someone help me? Thanks in Advance.

using UnityEngine;
using System.Collections;

public class PlayerMove : MonoBehaviour {
	public float speed;
	public float backSpeed;
	public float jumpVelocity;
	
	private bool isGrounded;
	
	void Awake(){
		Rigidbody rigid = GetComponent<Rigidbody>();
	}
	
	void Start(){
			
	}
	
	void Update(){
		if(Input.GetButtonDown("Jump")){
			if(isGrounded){
				rigidbody.AddForce(jumpVelocity, ForceMode.VelocityChange);
				isGrounded = false;
			}
		}
	}
	
	void FixedUpdate () {
		if(isGrounded){
			if(Input.GetKey(KeyCode.W)){
				rigidbody.AddForce(0f, 0f, backSpeed, ForceMode.Acceleration);
			}
			else if(Input.GetKey(KeyCode.S)){
				rigidbody.AddForce(0f, 0f, backSpeed, ForceMode.Acceleration);
			}
		}
	}
}

You declared jumpVelocity as float. Use it like this:

rigidbody.AddForce(jumpVelocity * Vector3.up, ForceMode.VelocityChange);