the floats wont be applied, Random.Range

{
float timeLeft;

    void start ()
    {
        timeLeft = Random.Range(5.0f, 10.0f);
    }
    void Update()
    {
        if (gameObject.GetComponent<Rigidbody>().isKinematic == false)
        {
            timeLeft -= Time.deltaTime;
            if (timeLeft < 0)
            {
                Destroy(gameObject);
            }
        }
    }
}

why wont the floats be applied to the timeLeft?

if you want unity to call a method to set timeLeft, try using the proper name…

start() is not the same as Start()