aws lambda calling external API showing error when it is working

Using a lambda calling an external API....Populating the POST with a JSON of an xml file.

API call: https:serverName/api/SyncPersonnelViaAwsApi/SapEaiCall

Hits the API function and returns the correct message 'Not latest version of file, update not performed'

However the lambda says it has failed.

    Response:
{
  "errorMessage": "\"{'response' : 'Not latest version of file, update not performed'}\""
}

this is all the data that is given in the logs...but this is the correct message postback...does anyone have any idea why it is still flagging as a fail?

(code below)

//// POST api/<controller>
  public string SapEaiCall([FromBody]string xmlFile)
    {
        string responseMsg = "Failed Import Active Directory User";

        if (string.IsNullOrEmpty(xmlFile))
        {
            responseMsg = "XML file is NULL";
        }

        if (responseMsg != "XML file is NULL")
        {
            xmlFile = RemoveFirstAndLastQuotes(xmlFile);

            if (!IsNewestVersionOfXMLFile(xmlFile))
            {
                responseMsg = "Not latest version of file, update not performed";
            }
            else
            {
                Business.PersonnelReplicate personnelReplicate = BusinessLogic.SynchronisePersonnel.BuildFromDataContractXml<Business.PersonnelReplicate>(xmlFile);
                bool result = Service.Personnel.SynchroniseCache(personnelReplicate);

                if (result)
                {
                    responseMsg = "Success Import Sap Cache User";
                }
            }
        }

        return "{\"response\" : \" " + responseMsg + " \" , \"isNewActiveDirectoryUser\" : \" false \"}";

    }

(lambda:)

   var querystring = require('querystring');
var https = require('https');
var fs = require('fs');

exports.handler = function(event, context) {

  const post_data = JSON.stringify('="xmlData"');

    // An object of options to indicate where to post to
    var post_options = {
        host: 'ServerName',
        protocol: 'https:',
       // port: '443',
        path: '/api/SyncPersonnelViaAwsApi/SapEaiCall',
        method: 'POST',
        json:post_data,
        headers: {
            'Content-Type': 'application/json',
            'Content-Length': post_data.length
        }
    };

   process.env.NODE_TLS_REJECT_UNAUTHORIZED = "0";
    var post_request = https.request(post_options, function(res) {
        var body = "";

        res.on('data', function(chunk)  {
            //chunk = '456';
            body += chunk;
        });

        res.on('end', function() {
            context.done(body);
        });

        res.on('error', function(e) {
            context.fail('error:' + e.message);
        });
    });

    // post the data
    post_request.write(post_data);
    post_request.end();
    console.log("posted data " +post_data);
};