Really, (after many tests) I cannot understand what the problem can be...
I disabled all auto mounting functions putting in comment all the rows of the file auto.master
and after I checked the service autofs :

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[angelo_dev@localhost ~]$ sudo systemctl status autofs -l
● autofs.service - Automounts filesystems on demand
   Loaded: loaded (/usr/lib/systemd/system/autofs.service; enabled)
   Active: active (running) since Sun 2018-01-07 09:01:06 IST; 1min 25s ago
  Process: 1205 ExecStart=/usr/sbin/automount $OPTIONS --pid-file /run/autofs.pid (code=exited, status=0/SUCCESS)
 Main PID: 1214 (automount)
   CGroup: /system.slice/autofs.service
           └─1214 /usr/sbin/automount --pid-file /run/autofs.pid

Jan 07 09:01:05 localhost.pluto automount[1214]: setautomntent: lookup(sss): setautomntent: No such file or directory
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Independently of the set up That I made,  the service doesn't work correctly...

what can be the cause ?   What can I try still ?

regards

Angelo