Problem when creating directory on SDCard. Android Studio

I have the following code, in which I want to create a directory (E4) in the SD external memory, however, when I run it I can not create it, and I also want it to be in / sdcard / documents /, but I put it on in ** / storage / emulated / 0 / ** Documents / and I can not find that folder anywhere

The code is as follows:

public static boolean isExternalStorageWritable() {
    String state = Environment.getExternalStorageState();
    if (Environment.MEDIA_MOUNTED.equals(state)) {
        return true;
    }
    else
    {
        Log.e("isExtStorageWritable", "Can´t write to Ext. Storage");
        return false;
    }
}

//Get the path to the Documents Folder
public static File getStorageDir(String folderName) {
    // Get the directory for the user's public documents directory.
    File documents = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOCUMENTS);
    //path will be storage/sdcard/documents/foldername
    File path = new File(documents, folderName);

    Log.e("getStorageDir", path.toString());

    if (!path.mkdirs()) {
        Log.e("getStorageDir", "Directory not created");
    }
    else
    {            Log.e("getStorageDir", "Directory OK");
    }
    return path;
}

The LogCat errors I have to check give me the following:

E / getStorageDir: / storage / emulated / 0 / Documents / E4

E / getStorageDir: Directory not created

I am running it on Android 6.0 (in a Cubot Echo Music), however that same code does work as I want on an Android 5.0 from Xiaomi. I'm new to Android Studio