No response ( Blank ) on Testing my GPTs Actions

i created a action schema to create a GPT that could perform a GET request to fetch a simple json string. But when i hit “Test”, no response is shown
gptactionissue picture

No api request is logged in access log of AWS EC2 Ubuntu 22.04 Nginx server.
The GET request works in postman.

My schema for reference

{
  "openapi": "3.0.0",
  "info": {
    "title": " API",
    "description": "API for services.",
    "version": "1.0.0"
  },
  "servers": [
    {
      "url": "https://apiwebsite.com/getg",
      "description": "API production server"
    }
  ],
  "paths": {
    "/": {
      "get": {
        "operationId": "getg",
        "x-openai-isConsequential": "true",
        "summary": "return the value from the api",
        "responses": {
          "200": {
            "description": "A string response",
            "content": {
              "text/plain; charset=utf-8": {
                "schema": {
                  "type": "string",
                  "example": "SAMPLE STRING"
                    }
                  }
                }
              }
            }
          }
        }
      }
    }
  }
}

More context, it uses some internal plugin instead of api itself ??? ( click on the screenshot to enlarge it )

Just publish the GPT, it works normally then