unable to login with puppeteer onBefore script

Hi, I'm new in backstopjs and puppeteer and I'm trying to login to my app through the onBefore.json file but it doesn't seem to work correctly. It even doesn't perform.

{
"id": "backstop_default",
"viewports": [
{
"label": "tablet",
"width": 1024,
"height": 768
}
],
"onBeforeScript": "puppet/onBefore.js",
"onReadyScript": "puppet/onReady.js",
"scenarios": [
{
"label": "BackstopJS Homepage",
"cookiePath": "backstop_data/engine_scripts/cookies.json",
"url": "myUrl",
"referenceUrl": "",
"readyEvent": "",
"readySelector": "",
"delay": 0,
"hideSelectors": [],
"removeSelectors": [],
"hoverSelector": "",
"clickSelector": "",
"postInteractionWait": 0,
"selectors": [],
"selectorExpansion": true,
"expect": 0,
"misMatchThreshold": 0.1,
"requireSameDimensions": true
}
],
"paths": {
"bitmaps_reference": "backstop_data/bitmaps_reference",
"bitmaps_test": "backstop_data/bitmaps_test",
"engine_scripts": "backstop_data/engine_scripts",
"html_report": "backstop_data/html_report",
"ci_report": "backstop_data/ci_report"
},
"report": [ "browser" ],
"engine": "puppeteer",
"engineOptions": {
"args": [ "--no-sandbox" ]
},
"asyncCaptureLimit": 5,
"asyncCompareLimit": 50,
"debug": true,
"debugWindow": true
}

Or maybe there is something wrong with my puppeteer script?

(async () => {
const puppeteer = require('puppeteer');
const browser = await puppeteer.launch();
const page = await browser.newPage({ waitUntil: 'domcontentloaded' });

const loginEl = await page.$('#tbxLogin');
const passwordEl = await page.$('#tbxPassword');
const btnSubmitEl = await page.$('#btnLogin');

loginEl.value = 'zzz';
passwordEl.value = 'yyy';
btnSubmitEl.click();
})();

Could you please help me?