SME 10 does not implement Active Directory. hence you can only use SME as a standalone samba server or need to enable nt1 protocol and max server protocol to nt1 to use nt4 style PDC. which is not encouraged as nt1 aka smb1 aka cifs is deprecated.
if you need AD DC you need to configure your own samba server and then you can use SME as secondary server.
a lot of the work to implement AD DC is available in bug tracker but has not made it for different reasons.
The most prevalent is lack of work force for doing the final
implementation, but few issues were also encountered like a major security one: losing posix ACL in SME samba if it was the AD DC while supported as a domain member, when no support for windows ACL is available in linux filesystems leading to different permissions to access to the same file depending if you access from samba, http, ftp, ssh or locally.
one workaround is to set you samba ad dc in a docker instance or a side vm and sme as domain member