Undo any change to /etc/sudoers. As said, it must not be modified by hand!
This is a possible solution. But the config parser has problems with sssd group names. We can’t adopt this solution until we fix Nethgui/Authorization/PolicyRule.php.
The other way is creating a Unix group, administrators and add some members to it. The builtin /usr/share/nethesis/NethServer/Authorization/base.json policy grants full access to members of administrators.
groupadd administrators
usermod -G administrators -a davidep@adnethesis.it
Now user davidep@adnethesis.it has full privileges from server-manager.