My default file :

upstream backend {
        server localhost:5000;
}

server {

        root /var/www/app/build;

        index index.html index.htm index.nginx-debian.html;
        server_name my_server_name;

        location / {
                try_files $uri $uri/ /index.html;
        }

        location /api/ {
                proxy_pass http://backend;
                proxy_set_header Host $host;
                proxy_set_header X-Real-IP $remote_addr;
                proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
                proxy_set_header X-Forwarded-Proto $scheme;

                # Timeout-uri pentru conexiunile cu backend-ul
                proxy_connect_timeout 15s;
                proxy_send_timeout 30s;
                proxy_read_timeout 30s;
                send_timeout 30s;

                # Protecția împotriva HTTP Flooding (Rate Limiting)

                add_header X-Frame-Options "DENY";
                add_header X-Content-Type-Options "nosniff";
                add_header X-XSS-Protection "1; mode=block";
                add_header Content-Security-Policy "default-src 'self'";
         }

Error in console
enter image description here

When I try to POST i have this error but when I use GET its ok

(I am beginner)

I tried to change default file many times but no results(