"Assets is not defined" error / WebGL trying to communicate with browser

Hello everyone !

I was looking for a way to communicate with the browser from a WebGL built when I found some answers in the official documentation. I followed the instructions, I got some errors I solved easily (mainly syntax errors -_-), but I get the following browser's error and I don't know where that come from :

 An error occurred running the Unity content on this page. See your browser JavaScript console for more info. The error was:
 ReferenceError: Assets is not defined

and this in the browser developement console :

 UnityLoader["51f77940c7b943726f70259e98b3bb81"]
 blob:https://localhost/6a14593e-adfc-3145-ba9e-3d6d1e8ddb18:18:1
 processAsmFrameworkJob/<
 https://localhost/unity3d/VisionneuseWebGL/Build/UnityLoader.js:95:7
 loadCode/script.onload
 https://localhost/unity3d/VisionneuseWebGL/Build/UnityLoader.js:26:7

Here are the scripts I wrote just for testing :

 Assets/Plugins/Interactions.jslib 
 mergeInto(LibraryManager.library,
   {
     SetBackground : function()
     {
       document.body.style.backgroundColor = "red";
     }
   });


 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using System.Runtime.InteropServices;
 
 public class GetInfos : MonoBehaviour
 {
     [DllImport("__Internal")]
     private static extern void SetBackground();
 
 
     public void SetDocBack() //this is called from a GUI Button
     {
         SetBackground();
     }
 }

My WebGL build doesn't even make the unity splash screen.

If you have some ideas please tell me, I'm stuck here...