Ngrok not tunneling properly Nginx
Any request coming in from ngrok, has the Host header set to the ngrok URL. The behaviour of nginx would be to try and match one of the server blocks in your configuration above, and default to the first one if no server_name matches the Host header.
However, I'm guessing there's another configuration file at /etc/nginx/conf.d/default.conf or /etc/nginx/sites-enabled/0-default which has a listen directive with default_server set. That will be catching these requests and serving the "Welcome to nginx!" page.
I suggest you look for that file, and remove it which should solve the issue.
However you could also simplify the above configuration and simply have:
server { listen 80; server_name localhost; location / { include proxy_params; proxy_pass http://unix:/home/datascience/chatbot-cima/chatbot.sock; }}Provided there's not another server block hiding somewhere else in the configuration with a directive like listen 80 default_server; then this should catch all requests.
For more info see: How nginx processes a request