Our SAMBA configuration is taking the users from os layer. Therefore samba-tool user list is not working as it is not configured this way. As a default value, we set valid users = dietpi. This will allow access using dietpi user only (with os password). You could try to adjust the password as follow:
smbpasswd -a dietpi
As well can you share following
journalctl -u nmbd -u smbd
systemctl status nmbd smbd
ss -tulpn | grep LISTEN