UnityException: RandomRangeInt is not allowed to be called from a MonoBehaviour constructor, call it in Awake or Start instead

In my Unity project, I'm trying to make a feature which every 5 seconds, a random number will be generated, and if the number equals 6, an enemy will be spawned. I tried to generate the number with UnityEngine.Random.Range(1,7) but I kept getting the following error:

UnityException: RandomRangeInt is not allowed to be called from a MonoBehaviour constructor (or instance field initializer), call it in Awake or Start instead. Called from MonoBehaviour 'spawnEnemy' on game object 'enemySpawn'. This is the code:

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

public class spawnEnemy : MonoBehaviour
{
    public GameObject enemyPrefab;
    public float spawnEnemySpeed = 5.0f;
    private float spawnTimer = 0.0f;
    int rng = 0;

    void Start()
    {
        rng = UnityEngine.Random.Range(1,7);
        Debug.Log(rng);
    }

    void Update()
    {
        spawnTimer += Time.deltaTime;
        if (spawnTimer >= spawnEnemySpeed && rng == 6)
        {
            Instantiate(enemyPrefab, transform.position, transform.rotation);
            spawnTimer = 0.0f;
            rng = UnityEngine.Random.Range(1,7);
            Debug.Log(rng);
        }
    }
}

I cannot find solution to this problem, most of the sources I found said that write the code in Start() and Awake(), but did not work for me even if I commented the code in the Update.

One of the source I found :