error in code saying: Assets\Grabbing.cs(19,45): error CS1503: Argument 1: cannot convert from 'UnityEngine.Quaternion' to 'float'

Question

I am working on a Ragdoll game and I watched a tutorial. But I got one error in a code and it says:

Assets\Grabbing.cs(19,45): error CS1503: Argument 1: cannot convert from 'UnityEngine.Quaternion' to 'float'

I don't get it.
This is my code:

void Update()
{
    Vector3 playerpos = new Vector3(cam.ScreenToWorldPoint(Input.mousePosition).x, cam.ScreenToWorldPoint(Input.mousePosition).y, 0);
    Vector3 difference = playerpos - transform.position;
    float rotationZ = Mathf.Atan2(difference.x, -difference.y) * Mathf.Rad2Deg;
    if (Input.GetKey(mousebutton))
    {
        rb.MoveRotation(Mathf.LerpAngle(rb.rotation, rotationZ, speed * Time.fixedDeltaTime));
    }
}