How to solve "http" directive is not allowed here in /etc/nginx/sites-enabled/default:1

Check your /etc/nginx/nginx.conf. If you are opening http directive there, and then opening it again into your sites-enabled/ folder conf file you will end up with nested http directive.

My response is about 9months late, but I have just experienced the same issue.

I decided to add my response because the last 2 answers only hinted at the solution and might still be confusing for some people.

A more straightforward answer is:

Remove the http declaration from your config above, since it most likely has been defined in your /etc/nginx/nginx.conf file (You can check that file to verify the claim).

So, your config should eventually look like this:

    upstream alert {
        # fail_timeout=0 means we always retry an upstream even if it failed
        # to return a good HTTP response
    
        # Unix domain servers
        server unix:/tmp/alert_1.sock fail_timeout=0;
        server unix:/tmp/alert_2.sock fail_timeout=0;
        server unix:/tmp/alert_3.sock fail_timeout=0;
        server unix:/tmp/alert_4.sock fail_timeout=0;
    
        # Unix domain sockets are used in this example due to their high performance,
        # but TCP/IP sockets could be used instead:
        # server 127.0.0.1:8081 fail_timeout=0;
        # server 127.0.0.1:8082 fail_timeout=0;
        # server 127.0.0.1:8083 fail_timeout=0;
        # server 127.0.0.1:8084 fail_timeout=0;
      }
      server {
        listen 80;
        client_max_body_size 4G;
    
        server_name myiphere;
    
        location / {
          proxy_set_header Host $http_host;
          proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
          proxy_redirect off;
          proxy_buffering off;
          proxy_pass http://alert;
        }
    
        location /static {
          # path for static files
          root /mylocation/static;
        }
    
      }

I had the same error with you met.I set a nginx config not directly in /ect/nginx/nginx.conf but in http{} in /ect/nginx/nginx.conf. So you can remove the /ect/nginx/nginx.conf and create a new /ect/nginx/nginx.conf with your own config code.