你设置了nginx配置文件来服务react文件在服务器块中，它是'位置/'。
因此，如果您尝试在'location /'块中添加proxy_pass设置，它将覆盖提供react文件的代码。Nginx会将请求代理到运行在localhost：3000上的后端服务器。

如何解决这个问题？

您必须在后端服务器中为该请求提供文件，或者添加新的位置块。
以下是添加新位置块示例

server {
    root /var/www/nikolsoborsocial/html;
    index index.html index.htm index.nginx-debian.html;

    server_name nikolsoborsocial nikolsoborsocial.org www.nikolsoborsocial.org;

    location / {
        try_files $uri $uri/ =404;
    }

    # Proxy Pass settings
    location /app {
        proxy_pass http://localhost:3000;
        include proxy_params;
    }

    # SSL configuration
    listen [::]:443 ssl ipv6only=on;
    listen 443 ssl;
    ssl_certificate /etc/letsencrypt/live/nikolsoborsocial.org/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/nikolsoborsocial.org/privkey.pem;
    include /etc/letsencrypt/options-ssl-nginx.conf;
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem;
}

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我更改了配置文件：

server {

    root /var/www/nikolsoborsocial/html;
    index index.html index.htm index.nginx-debian.html;

    server_name nikolsoborsocial nikolsoborsocial.org
    www.nikolsoborsocial.org;

    location /api/ {
        proxy_pass http://localhost:3000;
        # include proxy_params;
    }

    listen [::]:443 ssl ipv6only=on; # managed by Certbot
    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/nikolsoborsocial.org/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/nikolsoborsocial.org/privkey.pem; # managed by Certbot
    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot

}

server {
    if ($host = www.nikolsoborsocial.org) {
        return 301 https://$host$request_uri;
        } # managed by Certbot

        if ($host = nikolsoborsocial.org) {
            return 301 https://$host$request_uri;
            } # managed by Certbot

            listen 80;
            listen [::]:80;

            server_name nikolsoborsocial nikolsoborsocial.org
            www.nikolsoborsocial.org;

            return 404; # managed by Certbot

        }

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感谢您的指导@ACS
location /API/ -解决问题！