In Samba 4.18 the provision command fails (if no args are given) with:

# samba-tool domain provision
ERROR: No domain set!

Before 4.18 it automatically started the interactive mode and asked for realm, domain, dns, etc.

Workaround:
Use the --interactive parameter.
# samba-tool domain provision --interactive
Realm []:

I didn't check this in detail, but I assume that this has been introduced somewhere around commit ed78786989779f31d97de2aa81b06ef0c0ad7f39

In python/samba/netcmd/domain.py cmd_domain_provision() the following is broken, since len(self.raw_argv) is now 0, not 1:

if len(self.raw_argv) == 1:
    interactive = True