Validate path param in AWS API Gateway through OpenAPI spec

Question

I created a POST API with path parameter and request body through openAPI spec.

"/v2/{lang}/subject": {
  "post": {
    "parameters": [
        {
          "name": "lang",
          "in": "path",
          "required": true,
          "type": "string"
        },
        {
          "in": "body",
          "name": "subject",
          "required": true,
          "schema": {
            "$ref": "#/definitions/subject"
          }
        }
      ],
    "requestBody": {
      "content": {
        "application/json": {
          "schema": {
            "$ref": "#/components/schemas/subject"
          }
        }
      },
      "required": true
    },
    "x-amazon-apigateway-request-validator" : "all",
    .....
   }

I want to validate path param and request body at APIGW. Unfortunately, 1 test case is failing.

If I try accessing API with missing lang, ex: https://apigwsampleurl.com/v2/subject, AWS APIGW is throwing Missing Authentication Token error.

How this case should be handled in the configuration?