proxy_pass in Openresty

I want to check something in Lua and then proxy_pass in my target using Nginx code but does not work!

location /nt {
    default_type 'text/plain';
    content_by_lua_block{
        local res = ngx.location.capture("/download_server")
        if res.status == ngx.HTTP_OK then
          proxy_buffering off;
          proxy_set_header X-Real-IP $remote_addr;
          proxy_set_header Host $http_host;
          proxy_http_version 1.1;
          limit_req_status 444;
          proxy_pass http://127.0.0.1:3000;
        else
          ngx.say('Failed to process, please try again in some minutes.')
        end
    }
}

location /download_server {
    proxy_pass http://127.0.0.1:3000/nt;
}

Start by isolating the behavior of the /nt location and its content_by_lua_block, then compare it with the /download_server proxy_pass location. Capture the exact response and error output for both branches. Done means the configuration reliably proxies the target after a successful Lua check and returns the stated failure message otherwise.