Install application in root path

I am trying to install Mattermost from:

On the README it states that it can only be installed in the root path. But when I try to do so, I got an error like:

Warning: Unable to install the app in this location

There is no other app set as ‘default’ or installed into the root path.

Maybe you can’t install on root path if you have apps install on subpath. It is logic, cause your subpath apps could create conflict with the root apps.

The app on / need a directory called /files/ but your subpath apps is already installed on /files/…