You executed the command smbclient -L localhost -U% as root user? Do not use sudo.
If yes, please redo these two steps:
Add the root user to the SAMBA password database. The root user (alias: Administrator) will be our domain administrator. This account is needed to add new computers to the SAMBA domain.
smbpasswd -a root
Create the file /etc/samba/smbusers and add the line by executing:
echo "root = Administrator" > /etc/samba/smbusers
and try to query your server again.