Object not following mouse

Im trying to make the object follow my cursor. Im working in a 2d shooting game. There is no error before playing the scene but after playing it gives me this error:
“Object reference not set into an instance of an object”

This is my code for the object to follw my cursor:
public float offset;

public void Update()
    Vector3 difference = Camera.main.ScreenToWorldPoint(Input.mousePosition) - transform.position;
    float rotZ = Mathf.Atan2(difference.y, difference.x) * Mathf.Rad2Deg;
    transform.rotation = Quaternion.Euler(0f, 0f, rotZ + offset);

I am following a tutorial by blackthornprod:

when I downloaded and tried this code to the weapon of his character. The script works fine. But when I try to use the script in my own game it is not working. I suspect that it is because of a setting in a Object setting.

Knowledge about Unity: Beginner.

Unity is telling you that some object that your script is referencing, doesnt exists, thats the only thing I can find to help you, as I dont have more info in your game/script, hope it helped you a little.