What credentials did you provide? Could you show us some examples?
Did you try connecting with smbclient and reproduce the problem? Any error message from it?
IIRC the username provided to Samba must be different from the Unix (sssd) one! It does not have the @domain suffix. As said, the other required parameter is the workgroup/domain name.
This should not be a requirement because after “Start DC” button is pressed any package already present on the system is reconfigured.