With the new operating system, Apple has removed the activation of the ftp server from the "System Preferences" application.
Ftp is a very old and unsecure protocol, all communications are in plain text so they can be sniffed easily. It would be better using the sftp, but in some cases the ftp could be indispensable, for example if you have an already written procedure/script/program which makes use of the ftp, this was my case.
The ftp server hasn't been removed (yet) from the operating system, so I had only to find the right way to activate (and then deacitvate) it.
The launchctl command came in my help.
To activate the ftp server simply I opened a new terminal window and then I issued the following command:
$ sudo -s launchctl load -w /System/Library/LaunchDaemons/ftp.plist
At this point the ftp server was working fine and I could use my login credentials to access the ftp
To deactivate the ftp server I had to issue the following command:
$ sudo -s launchctl unload -w /System/Library/LaunchDaemons/ftp.plist
That's all, nothing more.