nginx reverse proxy multiple backends

@mohamnag's comment is right. proxy_pass is only allowed inside a location

See:

http://wiki.nginx.org/HttpProxyModule#proxy_pass

https://www.nginx.com/resources/admin-guide/reverse-proxy/

So the correct config would be

server {
    server_name client.example.com;

    location / {
        # app1 reverse proxy follow
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header Host $host;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_pass http://x.x.x.100:80;
    }
}

server {
    server_name client2.example.com;

    location / {
        # app2 reverse proxy settings follow
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header Host $host;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_pass http://x.x.x.101:80;
    }
}

You can match the different URLs with server {} blocks, then inside each server block, you'd have the reverse proxy settings.

Below, an illustration;

server {
  server_name client.example.com;

  # app1 reverse proxy follow
  proxy_set_header X-Real-IP $remote_addr;
  proxy_set_header Host $host;
  proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
  proxy_pass http://x.x.x.100:80;

}

server {
  server_name client2.example.com;

  # app2 reverse proxy settings follow
  proxy_set_header X-Real-IP $remote_addr;
  proxy_set_header Host $host;
  proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
  proxy_pass http://x.x.x.101:80;
}

Also, you may add further Nginx settings (such as error_page and access_log) as desired in each server {} block.