Yes... But regarding my initial issue, I've found the problem.
If added this function down to the template (html one) and then everything ok.

function toString (data) {
return JSON.stringify(data);
}

thanks for your support anyway.

Any idea regarding this error?

seems that I don't have enough privileges:

Sorry @bjrmatos , how can I attach a file here? do I have to post a downloading link?

I'm no sure what should I add on my global helpers.js file...Could you please let me know?
Here's my script code:

const request = require('request');

async function getMaApi() {
    const query = `{
  monitorData(machineId:"8af00d441c99a48aba6666a4"){
    trendValues(variables:["DT:addr_100","DT:addr_118","DT:addr_120"]since:1606842060000,until:1606842643000){
      label
      values{
        timestamp
        value
      }
      
    }
  }
}`;
   var postData = JSON.stringify({
            query,
            variables: null,
        })

    const options = {
        url: 'https://ecostruxure-machine-advisor.se.app/daas/graphql',
        method: 'POST',
        headers: {
            'Content-Type': 'application/json',
            'Authorization': 'Bearer 9RfMdTD8Nh3aMd8xN9cRSr5SKwTV6fCTpyQcQcA5'
            },
        body: postData
    };
    function doRequest(options){
        return new Promise(function(resolve,reject){
            request(options, function(err,res,body){
                resolve(body)
            })
        })
    }
    let response = await doRequest(options) //espera hasta que la promesa se resuelva
    //return JSON.parse(response).data.me.username - Ejemplo para acceder a un campo de la respuesta

    let trendValues = JSON.parse(response).data.monitorData.trendValues;

    for (let tagIndex = 0 ; tagIndex < trendValues.length; tagIndex++){
        for (let valueIndex = 0; valueIndex < trendValues[tagIndex].values.length; valueIndex ++){
            let timestampValue = trendValues[tagIndex].values[valueIndex].timestamp
            trendValues[tagIndex].values[valueIndex].timestamp = new Date(timestampValue*1000);
        }
    }
    let formattedResponse = trendValues;
    console.log(formattedResponse)
    return formattedResponse;
}

//Antes de renderizar hay que hacer algo. función JSReport
async function beforeRender(req, res) {
    req.data.trendValues = await getMaApi(); //en req.data añadimos toda la info que usaremos en el report. En machines
    //hemos asociado toda la respuesta
}
const nodemailer = require('nodemailer');
async function afterRender(req, res) {
    const transporter = nodemailer.createTransport({
      host: 'smtp.office365.com',
      port: 587,
      auth: {
          user: 'mnogueracolome@outlook.com',
          pass: '*****'
      }
    })  

    await transporter.sendMail({
      from: "mnogueracolome@outlook.com",
      to: "mnogueracolome@gmail.com",
      subject: "Sending email from node.js",
      text: "See the attached report",
      html: "<b>See the attached report</b>",
      /*attachments: [
      {  
        filename: 'Report.pdf',
        content: Buffer.from(res.content)
      }],*/
    })       
}

Hello,

I'm just starting with jsreport (also with JS language in fact...) and I've got this error when running my document and I don't know what could be wrong:

Error while executing templating engine. Missing helper: "toString"
Error: Missing helper: "toString"
at Object.<anonymous> (C:\Users\jsreportapp\node_modules\handlebars\dist\cjs\handlebars\helpers\helper-missing.js:19:13)
at Object.wrapper (C:\Users\jsreportapp\node_modules\handlebars\dist\cjs\handlebars\internal\wrapHelper.js:15:19)
at Object.eval [as main] (eval at createFunctionContext (C:\Users\jsreportapp\node_modules\handlebars\dist\cjs\handlebars\compiler\javascript-compiler.js:262:23), <anonymous>:21:110)
at main (C:\Users\jsreportapp\node_modules\handlebars\dist\cjs\handlebars\runtime.js:208:32)
at ret (C:\Users\jsreportapp\node_modules\handlebars\dist\cjs\handlebars\runtime.js:212:12)
at ret (C:\Users\jsreportapp\node_modules\handlebars\dist\cjs\handlebars\compiler\compiler.js:519:21)
at C:\Users\jsreportapp\node_modules\jsreport-handlebars\lib\handlebarsEngine.js:33:20
at Object.base.apply (C:\Users\jsreportapp\node_modules\vm2\lib/contextify.js:469:32)
at evaluate-template-engine.js:1:64
at Script.runInContext (vm.js:143:18)

Let me know if you need me to paste my code.

Thanks!