I'm trying to create a new volume. I'm in 4.1.2

I'm following these indications:

When I click the "add brick" button, I don't see anything in "Brick Directory" dropdown field and I cannot manuall input a directory name.

On the 3 nodes I already have formatted and mounted fs

The guide tells 

7. Click the Add Bricks button to select bricks to add to the volume. Bricks must be created externally on the Gluster Storage nodes.

What does it mean with "created externally"? 

The next step from os point would be volume creation but it is indeed what I would like to do from the gui...

Thanks,

Gianluca