Linux Nginx Reverse Proxy Does Not Serve Custom error.html

Question

I am running nginx/1.15.7 on Ubuntu 18.04. My setup is that I have nginx serving as a reverse proxy for a Python3 Flask application. I am trying to enable a single custom error.html for all major errors ( 401, 404, 502 ... etc. ). I have tried many examples found on Google, but I am unable to have nginx serve my custom error.html when I purposefully type in an invalid URI for my nginx server. I can, however, successfully view my error.html if I visit "/error.html.

How can I make this work?

My sites-enabled configuration file is shown below:

server {

   # Define the server name, IP address, and/or port of the server
   listen 80;

   # Define the specified charset to the “Content-Type” response header field
   charset utf-8;

   # Specify one error page for all errors
   error_page 400 /error.html;
   error_page 401 /error.html;
   error_page 403 /error.html;
   error_page 404 /error.html;
   error_page 500 /error.html;
   error_page 503 /error.html;
   error_page 504 /error.html;

   # Create 1 error page to rule them all
   location ~ /*error.html {
        try_files error.html @error;
        internal;
   }

   location @error {
       root /var/www/localhost/html;
   }

   # Configure NGINX to deliver static content from the specified folder for main webapp
   location /main-static {
      alias /home/root/MainWebApp/static;
   }

   # Favicon
   location = /favicon.ico {

      alias /home/root/MainWebApp/static/favicon.ico;

   }

   # This is the main WebApp
   location / {

       # Define the location of the proxy server to send the request to
       proxy_pass http://comp.local.com:9000;

       # Redefine the header fields that NGINX sends to the upstream server
       proxy_set_header Host $host;
       proxy_set_header X-Real-IP $remote_addr;
       proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
       proxy_buffering off;

       # Define the maximum file size on file uploads
       client_max_body_size 5M;

   }


}