What is problem? Assets/classs.cs(14,10): error CS0131: The left-hand side of an assignment must be a variable, a property or an indexer

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

public class classs : MonoBehaviour {

	public GameObject light;
	TOD_Sky ts; // this is script
	public int time=19;

	void Start () {
		if (ts.Cycle.Hour < time) {
			light.SetActive = true;
		}
	}

	
	// Update is called once per frame
	void Update () {
		
	}
}

SetActive is a method, not a variable. So this line makes no sense:

light.SetActive = true;

You probably want to do:

light.SetActive( true );

This will execute the method SetActive and pass “true” as parameter.