lanfeust:~# smbpasswd toto
New SMB password:
Retype new SMB password:
Failed to find entry for user karm.
Failed to modify password entry for user toto

  -a     This option specifies that the username following should be added to the local smbpasswd file,  with  the
              new  password typed (type <Enter> for the old password). This option is ignored if the username following
              already exists in the smbpasswd file and it is treated like a regular change password command. 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.