Discourse Running With Nginx Issue

Unless you are deeply familiar with reverse proxies shenanigans, I recommend you to to not stray away from the beaten path and follow only the official install guide.

There are some community guides about allowing multiple different software in the same host, sharing a single reverse proxy at the OS level, but that is prone to breakage and not worth your time unless you are on a really tight budget or working on stuff like that for a living.