504 Gateway Time-out

When I was trying to install nextcloud, I downloaded the latest version, unzip it and create a config file for nginx following the official website. When I visit the nginx welcome page(https://ipv6.sciencekill.top), it works well. But when I visit nextcloud(https://ipv6.sciencekill.top/nextcloud), it threw 504 Gateway Time-out. I have give the directory 777 privileges. And here is the config file(nextcloud is at /var/www/nextcloud)

upstream php-handler {
    server 127.0.0.1:9000;
    #server unix:/var/run/php/php7.4-fpm.sock;
}

server {
    listen 80;
    listen [::]:80;
    server_name ipv6.sciencekill.top;
    # enforce https
    return 301 https://$server_name:443$request_uri;
}

server {
    listen 443 ssl http2;
    listen [::]:443 ssl http2;
    server_name ipv6.sciencekill.top;

    # Use Mozilla's guidelines for SSL/TLS settings
    # https://mozilla.github.io/server-side-tls/ssl-config-generator/
    # NOTE: some settings below might be redundant
    ssl_certificate /etc/letsencrypt/live/sciencekill.top/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/sciencekill.top/privkey.pem;

    # Add headers to serve security related headers
    # Before enabling Strict-Transport-Security headers please read into this
    # topic first.
    #add_header Strict-Transport-Security "max-age=15768000; includeSubDomains; preload;" always;
    #
    # WARNING: Only add the preload option once you read about
    # the consequences in https://hstspreload.org/. This option
    # will add the domain to a hardcoded list that is shipped
    # in all major browsers and getting removed from this list
    # could take several months.
    add_header Referrer-Policy "no-referrer" always;
    add_header X-Content-Type-Options "nosniff" always;
    add_header X-Download-Options "noopen" always;
    add_header X-Frame-Options "SAMEORIGIN" always;
    add_header X-Permitted-Cross-Domain-Policies "none" always;
    add_header X-Robots-Tag "none" always;
    add_header X-XSS-Protection "1; mode=block" always;

    # Remove X-Powered-By, which is an information leak
    fastcgi_hide_header X-Powered-By;

    # Path to the root of your installation
    root /var/www;

    location = /robots.txt {
        allow all;
        log_not_found off;
        access_log off;
    }

    # The following 2 rules are only needed for the user_webfinger app.
    # Uncomment it if you're planning to use this app.
    #rewrite ^/.well-known/host-meta /nextcloud/public.php?service=host-meta last;
    #rewrite ^/.well-known/host-meta.json /nextcloud/public.php?service=host-meta-json last;

    # The following rule is only needed for the Social app.
    # Uncomment it if you're planning to use this app.
    #rewrite ^/.well-known/webfinger /nextcloud/public.php?service=webfinger last;

    location = /.well-known/carddav {
      return 301 $scheme://$host:$server_port/nextcloud/remote.php/dav;
    }
    location = /.well-known/caldav {
      return 301 $scheme://$host:$server_port/nextcloud/remote.php/dav;
    }

    location /.well-known/acme-challenge { }

    location ^~ /nextcloud {

        # set max upload size
        client_max_body_size 512M;
        fastcgi_buffers 64 4K;

        # Enable gzip but do not remove ETag headers
        gzip on;
        gzip_vary on;
        gzip_comp_level 4;
        gzip_min_length 256;
        gzip_proxied expired no-cache no-store private no_last_modified no_etag auth;
        gzip_types application/atom+xml application/javascript application/json application/ld+json application/manifest+json application/rss+xml application/vnd.geo+json application/vnd.ms-fontobject application/x-font-ttf application/x-web-app-manifest+json application/xhtml+xml application/xml font/opentype image/bmp image/svg+xml image/x-icon text/cache-manifest text/css text/plain text/vcard text/vnd.rim.location.xloc text/vtt text/x-component text/x-cross-domain-policy;

        # Uncomment if your server is build with the ngx_pagespeed module
        # This module is currently not supported.
        #pagespeed off;

        location /nextcloud {
            rewrite ^ /nextcloud/index.php;
        }

        location ~ ^\/nextcloud\/(?:build|tests|config|lib|3rdparty|templates|data)\/ {
            deny all;
        }
        location ~ ^\/nextcloud\/(?:\.|autotest|occ|issue|indie|db_|console) {
            deny all;
        }

        location ~ ^\/nextcloud\/(?:index|remote|public|cron|core\/ajax\/update|status|ocs\/v[12]|updater\/.+|oc[ms]-provider\/.+|.+\/richdocumentscode\/proxy)\.php(?:$|\/) {
            fastcgi_split_path_info ^(.+?\.php)(\/.*|)$;
            set $path_info $fastcgi_path_info;
            try_files $fastcgi_script_name =404;
            include fastcgi_params;
            fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
            fastcgi_param PATH_INFO $path_info;
            fastcgi_param HTTPS on;
            # Avoid sending the security headers twice
            fastcgi_param modHeadersAvailable true;
            # Enable pretty urls
            fastcgi_param front_controller_active true;
            fastcgi_pass php-handler;
            fastcgi_intercept_errors on;
            fastcgi_request_buffering off;
        }

        location ~ ^\/nextcloud\/(?:updater|oc[ms]-provider)(?:$|\/) {
            try_files $uri/ =404;
            index index.php;
        }

        # Adding the cache control header for js, css and map files
        # Make sure it is BELOW the PHP block
        location ~ ^\/nextcloud\/.+[^\/]\.(?:css|js|woff2?|svg|gif|map)$ {
            try_files $uri /nextcloud/index.php$request_uri;
            add_header Cache-Control "public, max-age=15778463";
            # Add headers to serve security related headers  (It is intended
            # to have those duplicated to the ones above)
            # Before enabling Strict-Transport-Security headers please read
            # into this topic first.
            #add_header Strict-Transport-Security "max-age=15768000; includeSubDomains; preload;" always;
            #
            # WARNING: Only add the preload option once you read about
            # the consequences in https://hstspreload.org/. This option
            # will add the domain to a hardcoded list that is shipped
            # in all major browsers and getting removed from this list
            # could take several months.
            add_header Referrer-Policy "no-referrer" always;
            add_header X-Content-Type-Options "nosniff" always;
            add_header X-Download-Options "noopen" always;
            add_header X-Frame-Options "SAMEORIGIN" always;
            add_header X-Permitted-Cross-Domain-Policies "none" always;
            add_header X-Robots-Tag "none" always;
            add_header X-XSS-Protection "1; mode=block" always;

            # Optional: Don't log access to assets
            access_log off;
        }

        location ~ ^\/nextcloud\/.+[^\/]\.(?:png|html|ttf|ico|jpg|jpeg|bcmap|mp4|webm)$ {
            try_files $uri /nextcloud/index.php$request_uri;
            # Optional: Don't log access to other assets
            access_log off;
        }
    }
}

Did you setup and start PHP FPM? That’s what is not responding.

Try (change it before to your php-Version)

sudo service php8.1-fpm restart;
php -i | grep opcache.interned_strings_buffe;
php -i | grep opcache.max_accelerated_files;
php -i | grep opcache.memory_consumption;

Thanks for your advice. But after I do as your advice, the website now threw 502 Bad Gateway(you can check it at the website above in the question). By the way, I am using ddns(public ipv6),does that matter?