When working on a very secure environment, the Joomla FTP Layer may be required.
To allow the installation of a slave sites and the creation of the "configuration.php" file into the "/multisites/xxx" where xxx is the site ID, a patch is required to tell Joomla that this directory is the one where is installed Joomla.

When the slave sites is installed, its Global Configuration must be changed to restore the master website directory in aim to allow the installation of the extensions.