Couple things to try.
Stop nodebb and run ./nodebb build and try again.
Also run ./nodebb plugins to check active plugins and see if persona is active.
@audreybest That depends on your Nginx configuration. When you only pass the traffic to your NodeBB instance, it will be served by the internal NodeBB HTTP server (through Nginx). When you add pointers to the locations of the static files on your filesystem, then it will be served by Nginx directly.
Using the configuration as in the link of @pichalite will be the second case.