Timed repeat.

Use code tags:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class timertest : MonoBehaviour
{
    public GameObject Timer;

    public class timer : MonoBehaviour
    {
        public Text timerLabel;
        private float time;

        private void Start()
        {
            time = timerLabel;
        }

        void Update()
        {
            time += Time.deltaTime;

            var minutes = time / 60;
            var seconds = time % 60;
            var fraction = (time * 100) % 100;

            timerLabel.text = string.Format("{0:00} : {1:00} : {2:000}", minutes, seconds, fraction);
        }
    }
}

And for starters… you have a script nested inside another script. Though technically valid C#, it doesn’t meet the requirements of how Unity expects MonoBehaviours to be written (all MonoBehaviours/components should be contained in its own .cs file, and the filename should match the class name).

Also, the only line that throws a compiler error for me is this one:

time = timerLabel

And that’s because you’re attempting to set the float time with a Text object timerLabel.