After following the passenger installation instructions from https://www.phusionpassenger.com/library/install/nginx/install/oss/el7/ I stubled upon an error:

nginx: [emerg] unknown directive "passenger_root" in /etc/nginx/conf.d/passenger.conf:1

Which means that passenger module was probably not loaded correctly.

The fix

Find nginx module loading configuration

First check where your nginx looks for module configs. It’s usually written in nginx.conf and it may load all conf files from module-enabled or modules.conf.d directory, you can just grep your config to find out:

cd /etc/nginx
grep module *

Example output:

nginx.conf:include /etc/nginx/modules-enabled/*.conf;

Find passenger module location

You have to create a new conf file there for loading the passenger module, but first find the module location:

find / -name ngx_http_passenger_module.so

Example output:

/usr/lib64/nginx/modules/ngx_http_passenger_module.so

Tell nginx to load the passenger module

Now create new module configuration for phusion passenger and add above module there:

# cat /etc/nginx/modules-enabled/phusion-passenger.conf
load_module /usr/lib64/nginx/modules/ngx_http_passenger_module.so;

Restart Nginx

sudo systemctl restart nginx