Error
Nginx will not start due to the following error on Ubuntu 18:
nginx: [emerg] unknown directive "passenger_enabled" in /etc/nginx/sites-enabled/default:25
nginx: configuration file /etc/nginx/nginx.conf test failed
Resolution
Install Passenger + Nginx module
apt-get install -y libnginx-mod-http-passenger
Add a line on top of /etc/nginx/nginx.conf
load_module /usr/lib/nginx/modules/ngx_http_passenger_module.so;
Restart Nginx
service nginx restart