Unity: The variable rb of Movement has not been assigned

Question

I'm new to Unity and I'm following this tutorial. The error I face when I try to execute to code, which is supposed to move the player left and right is:

UnassignedReferenceException: The variable rb of Movement has not been assigned.
You probably need to assign the rb variable of the Movement script in the inspector.
UnityEngine.Rigidbody.AddForce (UnityEngine.Vector3 force, UnityEngine.ForceMode mode) <0x3cc39790 + 0x00062> in <31353b3f2d2e434595cbfe07fbec4072>:0
UnityEngine.Rigidbody.AddForce (System.Single x, System.Single y, System.Single z) (at C:/buildslave/unity/build/Runtime/Dynamics/ScriptBindings/Dynamics.bindings.cs:176)
Movement.FixedUpdate () (at Assets/Movement.cs:17)

How to fix it because the player does not move?

Movement script:

   using UnityEngine;

public class Movement : MonoBehaviour
{

    // This is a reference to the Rigidbody component called "rb"
    public Rigidbody rb;
    public float forwardForce = 2000f;  // Variable that determines the forward force
    public float sidewaysForce = 500f;  // Variable that determines the sideways force

    // We marked this as "Fixed"Update because we
    // are using it to mess with physics.
    void FixedUpdate()
    {
        // Add a forward force
        rb.AddForce(0, 0, forwardForce * Time.deltaTime);

        if (Input.GetKey("d"))  // If the player is pressing the "d" key
        {
            // Add a force to the right
            rb.AddForce(sidewaysForce * Time.deltaTime, 0, 0);
        }

        if (Input.GetKey("a"))  // If the player is pressing the "a" key
        {
            // Add a force to the left
            rb.AddForce(-sidewaysForce * Time.deltaTime, 0, 0);
        }
    }
}

Answer 1

You haven't set anything to rb. In the inspector, you can select a rigidbody to work with, by clicking on "this" (check picture)