Check your nginx config file for errors using this command:

sudo nginx -t

What does it output?

it said like this

nginx: [emerg] "location" directive is not allowed here in /etc/nginx/sites-enabled/default:84
nginx: configuration file /etc/nginx/nginx.conf test failed
root@tes:~# [emerg] "location" directive is not allowed here in /etc/nginx/sites-enabled/default:84
[emerg]: command not found

here is /etc/nginx/nginx.conf file

user www-data;
worker_processes auto;
worker_rlimit_nofile 100000;
pid /run/nginx.pid;

events {
        worker_connections 4096;
        multi_accept on;
}

http {
        ##
        # EasyEngine Settings
        ##

        sendfile on;
        tcp_nopush on;
        tcp_nodelay on;
        keepalive_timeout 30;
        types_hash_max_size 2048;

        server_tokens off;
        reset_timedout_connection on;
        add_header X-Powered-By "EasyEngine 3.7.5";
        add_header rt-Fastcgi-Cache $upstream_cache_status;

        # Limit Request
        limit_req_status 403;
        limit_req_zone $binary_remote_addr zone=one:10m rate=1r/s;

        # Proxy Settings
        # set_real_ip_from      proxy-server-ip;
        # real_ip_header        X-Forwarded-For;

        fastcgi_read_timeout 300;
        client_max_body_size 100m;

        ##
        # SSL Settings
        ##

        ssl_session_cache shared:SSL:20m;
        ssl_session_timeout 10m;
        ssl_prefer_server_ciphers on;
        ssl_ciphers "EECDH+AESGCM:EDH+AESGCM:AES256+EECDH:AES256+EDH";
        ssl_protocols TLSv1 TLSv1.1 TLSv1.2;

        ##
        # Basic Settings
        ##
        # server_names_hash_bucket_size 64;
        # server_name_in_redirect off;

        include /etc/nginx/mime.types;
        default_type application/octet-stream;

        ##
        # Logging Settings
        ##

        access_log /var/log/nginx/access.log;
        error_log /var/log/nginx/error.log;

        # Log format Settings
        log_format rt_cache '$remote_addr $upstream_response_time $upstream_cache_status [$time_local] '
        '$http_host "$request" $status $body_bytes_sent '
        '"$http_referer" "$http_user_agent"';

        ##
        # Gzip Settings
        ##

        gzip on;
        gzip_disable "msie6";

        gzip_vary on;
        gzip_proxied any;
        gzip_comp_level 6;
        gzip_buffers 16 8k;
        gzip_http_version 1.1;
        gzip_types
            application/atom+xml
            application/javascript
            application/json
            application/rss+xml
            application/vnd.ms-fontobject
            application/x-font-ttf
            application/x-web-app-manifest+json
            application/xhtml+xml
            application/xml
            font/opentype
            image/svg+xml
            image/x-icon
            text/css
            text/plain
            text/x-component
            text/xml
            text/javascript;

        ##
        # Virtual Host Configs
        ##

        include /etc/nginx/conf.d/*.conf;
        include /etc/nginx/sites-enabled/*;
}


#mail {
#       # See sample authentication script at:
#       # http://wiki.nginx.org/ImapAuthenticateWithApachePhpScript
#
#       # auth_http localhost/auth.php;
#       # pop3_capabilities "TOP" "USER";
#       # imap_capabilities "IMAP4rev1" "UIDPLUS";
#
#       server {
#               listen     localhost:110;
#               listen     localhost:110;
#               protocol   pop3;
#               proxy      on;
#       }
#
#       server {
#               listen     localhost:143;
#               protocol   imap;
#               proxy      on;
#       }
#}

and here is /etc/nginx/sites-enabled/default

##
# You should look at the following URL's in order to grasp a solid understanding
# of Nginx configuration files in order to fully unleash the power of Nginx.
# http://wiki.nginx.org/Pitfalls
# http://wiki.nginx.org/QuickStart
# http://wiki.nginx.org/Configuration
#
# Generally, you will want to move this file somewhere, and start with a clean
# file but keep this around for reference. Or just disable in sites-enabled.
#
# Please see /usr/share/doc/nginx-doc/examples/ for more detailed examples.
##

# Default server configuration
#
server {
        listen 80 default_server;
        listen [::]:80 default_server;

        # SSL configuration
        #
        # listen 443 ssl default_server;
        # listen [::]:443 ssl default_server;
        #
        # Self signed certs generated by the ssl-cert package
        # Don't use them in a production server!
        # include snippets/snakeoil.conf;
        #
        # ssl_protocols TLSv1 TLSv1.1 TLSv1.2; # don’t use SSLv3 ref: POODLE
        # ssl_ciphers HIGH:!aNULL:!MD5;
        # ssl_prefer_server_ciphers on;

        root /var/www/html;

        # Add index.php to the list if you are using PHP
        index index.html index.htm index.nginx-debian.html;

        server_name _;

        location / {
                # First attempt to serve request as file, then
                # as directory, then fall back to displaying a 404.
                try_files $uri $uri/ =404;
        }

        # pass the PHP scripts to FastCGI server listening on 127.0.0.1:9000
        #
        #location ~ \.php$ {
        #       include snippets/fastcgi-php.conf;
        #
        #       # With php5-cgi alone:
        #       fastcgi_pass 127.0.0.1:9000;
        #       # With php5-fpm:
        #       fastcgi_pass unix:/var/run/php5-fpm.sock;
        #}

        # deny access to .htaccess files, if Apache's document root
        # concurs with nginx's one
        # concurs with nginx's one
        #
        #location ~ /\.ht {
        #       deny all;
        #}
}


# Virtual Host configuration for example.com
#
# You can move that to a different file under sites-available/ and symlink that
# to sites-enabled/ to enable it.
#
#server {
#       listen 80;
#       listen [::]:80;
#
#       server_name example.com;
#
#       root /var/www/example.com;
#       index index.html;
#
#       location / {
#               try_files $uri $uri/ =404;
#       }
#}
        location /wp-content/ {
                alias /mnt/volume-sfo2-01/wp-content;
        }

You have defined the wp-content block outside of the server block, in the /etc/nginx/sites-enabled/default file. That causes nginx to fail.
Move it to the first server block, for example after location / block (you can put it anywhere as long as it's in the first server block):

...
server {
...
       server_name _;

        location / {
                # First attempt to serve request as file, then
                # as directory, then fall back to displaying a 404.
                try_files $uri $uri/ =404;
        }

       location /wp-content/ {
                alias /mnt/volume-sfo2-01/wp-content;
        }
...
}

Then, try to run nginx -t again, and if everything is okay, restart Nginx to put changes in the effect: sudo systemctl restart nginx.

thanks, it's worked..
but when I upload an image to my site, it's still saved in example.com/htdocs/wp-content/....
it's not saved in /mnt/vol.../wp-content
is there something wrong? or I was missunderstood about aliases?