Flutter: IAM Authorisation is throwing 403 with Amplify API post request

I'm trying to fetch the data from DynamoDB using API Gateway + Lambda from flutter APP.

I'm using AWS Amplify plugins for the same

Link to AWS docs: https://docs.amplify.aws/lib/restapi/getting-started/q/platform/flutter#initialize-amplify-api

When authorization is set to None I'm able to fetch the data successfully

But when I set the authorization type to IAM for API gateway I'm facing the below mentioned error.

post call failed: ApiException(message: The HTTP response status code is [403]., recoverySuggestion: 
I/flutter ( 6554):                     The metadata associated with the response is contained in the HTTPURLResponse.
I/flutter ( 6554):                     For more information on HTTP status codes, take a look at
I/flutter ( 6554):                     https://en.wikipedia.org/wiki/List_of_HTTP_status_codes
I/flutter ( 6554):                     , underlyingException: null)

My Code:

  Future<List<ChapterData>> getChapters(String subjectID) async {
    String postRequest = '{\"subject_id\":\"' + subjectID + '\"}';
    print(postRequest);
    try {
      RestOptions options = RestOptions(
          path: "/getChapterList",
          body: Uint8List.fromList(postRequest.codeUnits),
          apiName: "getChapterList");
      RestOperation restOperation = Amplify.API.post(restOptions: options);
      RestResponse response = await restOperation.response;
      var chapterData = ChapterData.fromJson(
        json.decode(new String.fromCharCodes(response.data)));
      return chapterData;
      } on ApiException catch (e) {
        print('post call failed: $e');
      }
  }
}

Is there anything I missed adding to the post request since it is failing only when I set the authorization type to IAM.

I didn't find any documentation related to this for flutter/dart. Also, I'm new to flutter

Thanks in Advance