Read and Write file on streamingAssetsPath

Now my problem is that i don't know how to write the file on mobile cause I do it like this on the standalone

You can't save to this location. Application.streamingAssetsPath is read-only. It doesn't matter if it works on the Editor or not. It is read only and cannot be used to load data.

Reading data from the StreamingAssets:

IEnumerator loadStreamingAsset(string fileName)
{
    string filePath = System.IO.Path.Combine(Application.streamingAssetsPath, fileName);

    string result;

    if (filePath.Contains("://") || filePath.Contains(":///"))
    {
        WWW www = new WWW(filePath);
        yield return www;
        result = www.text;
    }
    else
    {
        result = System.IO.File.ReadAllText(filePath);
    }

    Debug.Log("Loaded file: " + result);
}

Usage:

Let's load your "datacenter.json" file from your screenshot:

void Start()
{
    StartCoroutine(loadStreamingAsset("datacenter.json"));
}

Saving Data:

The path to save a data that works on all platform is Application.persistentDataPath. Make sure to create a folder inside that path before saving data to it. The StreamReader in your question can be used to read or write to this path.

Saving to the Application.persistentDataPath path:

Use File.WriteAllBytes

Reading from the Application.persistentDataPath path

Use File.ReadAllBytes.

See this post for a complete example of how to save data in Unity.