how can i write this properly

 public GameObject[] streetLights; // This is an array of GameObjects
 
 void Start(){
     streetLights = GameObject.FindGameObjectsWithTag("StreetLights");
 }

Remember to declare the type of the variable before you declare the name of it.

You can use UnityEvents to handle this

 using UnityEngine;
 using UnityEvent.Events;
 
 public class StreetLightScript: MonoBehaviour
 {
     public GameObject lightInChild;
     public static UnityEvent OnToggleStreetlights = new UnityEvent();
     
     void OnEnable()
     {
         OnToggleStreeLights.AddListener(ToggleLight);
     }
     void OnDisable()
     {
         OnToggleStreeLights.RemoveListener(ToggleLight);
     }
     void ToggleLight()
     {
         lightInChild.SetActive(!lightInChild.activeSelf);
     }  
 }

Since I assume you're trying to swap the object's active state we can't just simply use OnEnable and OnDisable on the object itself as usual. Thus this script will go on a parent/container object with the Streetlight as a child. then pull the reference of the child object into the lightInChild field so that the script has direct access to it.

then in any script anywhere you can simply call

 StreetLightScript.OnToggleStreetlights.Invoke();

and all lights currently in the scene will toggle their active state. And you never need to use the Find functions.

Also you can expand to having other, more specific events not just toggle. Such as OnIssueAllLightsOn.invoke() and OnIssueAllLightsOff.invoke()

Here's a simple script that will get all the lights in your scene with the tag "StreetLights" and a function that allows you to turn them all off.

 public GameObject[] streetlights;
 
     void Start () 
     {
         streetlights = GameObject.FindGameObjectsWithTag("StreetLights");
     }
     
     void TurnOffAllLights()
     {
         foreach(GameObject light in streetlights)
         {
             light.GetComponent<Light>().enabled = false;
         }
     }