I had that same bug with another jail of mine. It was fixable.
May not be the same issue exactly but for my issue this forum helped. https://forums.freenas.org/index.php?threads/cannot-use-pkg-install-on-new-plugin-jail-freenas-9-2-1.18319/
This advice in particular:
edit /usr/local/etc/pkg/repos/FreeBSD.conf
make the url http:// only (not pkg+http)
after this “pkg update”
"upgrade pkg"
edit the FreeBSD.conf file again and re-add the pkg+http part
apologies i can’t be more help i fixed it quite a while ago.