Can't create directory with mkdirs() - android

I try to create a directory and use the following code:

boolean success = true;
String rootDirectory = Environment.getExternalStorageDirectory().toString();
folder = new File(rootDirectory, "Directory");
if(!(folder.exists())) {
     success = folder.mkdirs();
} else {

}

if(success) {
   Toast.makeText(getActivity().getApplicationContext(), "DIR created", Toast.LENGTH_SHORT).show();     
} else {
     Toast.makeText(getActivity().getApplicationContext(), "DIR not created successfully", Toast.LENGTH_SHORT).show();
}

I also searched for the folder if it was created, there is none.

Permissions are granted:

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

I also tried to ask for permission during runtime, it seems like the app has got the permission, therefore it cannot be the problem.

Some months ago I created another application and used identical code and the identical Sdk version, still it does not work with this one. I get "DIR not created successfully" and I do not know why, please help me figure out why I cannot create the directory.

Use code below

File directory = new File(Environment.getExternalStorageDirectory() + java.io.File.separator +"Directory");
if (!directory.exists())
        Toast.makeText(getActivity(), 
          (directory.mkdirs() ? "Directory has been created" : "Directory not created"),
        Toast.LENGTH_SHORT).show(); 
else
     Toast.makeText(getActivity(), "Directory exists", Toast.LENGTH_SHORT).show();

For Android API 23 (Marshmallow) and greater, we have to allow dangerous permissions otherwise our code will not work as we expected.