Running recent 12-CURRENT (FreeBSD 12.0-ALPHA2 #440 r338208: Wed Aug 22 20:23:17 CEST 2018 amd64) and installing the base jail system from /usr/src, in recent times the installation of a new jail is incomplete as the newly installed jail lacks in etc/rc! So starting the jail fails. It seems that there are other rc-scripts missing: rc.subr isn't available in a newly created jail's etc folder, for instance. The last time I installed a jail was around 1 1/2 months ago and nothing changed so far in the basic configuration. So I was wondering if this is some side effect of recent changes in 12-CURRENT. Ports tree is at Revision: 477799, the ports on the system are up to date as of today.
I checked again on 12-ALPHA3 on a complete new installation of the base jail system (using ZFS, using either /usr/src or another source-path for the base system). After the (reported successful) installation of the base jail infrastructure, it seems that newjail/etc is now missing some important rc-scripts crucial for starting the jail: rc and rc.subr are missing from a first glimpse of investigation. I recall faintly that there have been made changes to the source tree of 12-CURRENT recently where rc.scripts have been moved from their legacy source, /usr/src/etc to /usr/src/sbin/init/rc.* (or, more generic, SRCDIR/sbin/init/rc.*). I guess ezjail does not fit this recent change and I have not dug deeper into that subject.
*** This bug has been marked as a duplicate of bug 230690 ***