The smbpasswd manual state the following:Note that the default passdb backends require the user to already  exist in the system password file (usually /etc/passwd), else the  request to add the user will fail.Source: https://www.samba.org/samba/docs/man/manpages-3/smbpasswd.8.htmlMy question is that which are the default passdb backends?smbpasswd?tdbsam?ldapsam?
default passdb backends in Samba
samba;samba4
default file to store pass is trival database file /var/lib/samba/passdb.tdbyou can change in by passdb backend option in smb.conf.There is more info about password backends: https://www.samba.org/samba/docs/man/Samba-HOWTO-Collection/passdb.htmldefault samba need to user exist in system, and check if it exist in /etc/passwd but this file don't contain passwords.