I don’t know where you got those instructions from, but I have mentioned several times in several different places in these forums that you must run from the ubuntu_bionic branch. I have no idea how to switch branches, but here it is easier if you only download the ubuntu_bionic branch. You forgot the --single-branch -b ubuntu_bionic.
So here is the instructions you should have followed, @blades :
$ git clone --single-branch -b ubuntu_bionic --depth=1 https://github.com/mail-in-a-box/mailinabox.git
$ cd mailinabox
# bash setup/start.sh