Below is the code that I'm using
String root = Environment.getExternalStorageDirectory().getAbsolutePath().toString();
File filepathname = new File(root+"/newfolder");
Toast.makeText(this,"directory created", Toast.LENGTH_SHORT).show();
Toast.makeText(this,"directory not created", Toast.LENGTH_SHORT).show();
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
In my opinion you dont have rights to create folder. If your application target is 23 or above you have to read about new runtime permissions.
Here you can find official documentation about it.