Try the Ubuntu docs instead of the Debian docs. They're more up to date.
NodeBB installation in a directory
I'm trying to install NodeBB on https://myip/community.
My current configs are as follows:
First of all your NGINX config is incorrect. You want to access the forum under /community, but are using /forum in it.
This is a simplified version of my NGINX config:
And here my config.json: