Here is what I do and it works every time on Ubuntu 18.04 after of course using the apt-get update and apt-get upgrade.
To Install v12 on Ubuntu 18.04 (from def_user account)
sudo -H apt-get install python3-minimal
sudo -H apt-get install python3-setuptools
sudo -H apt-get install build-essential
sudo -H apt-get install redis-server
wget https://raw.githubusercontent.com/frappe/bench/master/playbooks/install.py
sudo -H python3 install.py --production --user [def_user]
sudo nano /etc/hostname (change hostname here to URL)
sudo nano /etc/hosts (make sure hostname here is URL tied to ip address)
sudo nano /etc/cloud/cloud.cfg (change line “preserve_hostname” from false to true)
after install – edit ~/.bench/bench/config/nginx.py (change “Unicode” to “bytes”)
I installed this 2 times yesterday on 2 fresh VPS servers. I do all of this BEFORE I ever login to the Administrator and finish the setup wizard.
One of the installations was done to make an upgrade path from a v11 server. See this post to know how to also do that upgrade: (and yes, it was a success also 
)
Hope this helps. 
BKM