I'm very new to making chrome extension, not sure why but my javascript code returns on start

Cannot read properties of undefined

(reading cells), but in console everything works fine.

if(document.getElementById('pricelist')){
    var elementTable = document.getElementById('pricelist').tBodies[0];

    for (let i = 0; i < 25; i++) {
        for (let x = 1; x < 14; x++){
            let value = parseInt(elementTable.rows[i].cells[x].innerText);
            let mainValue = parseInt(elementTable.rows[i].cells[2].innerText);
            let diference = mainValue - value;
            let percentage = (diference/steamValue)*100;
            let newValue = value + "(" + percentage + ")";
            document.getElementById('pricelist').tBodies[0].rows[i].cells[x].innerHTML = newValue
        }    
    }
}

json:

{
    "name": "calculator",
    "version": "1.0",
    "manifest_version": 2,
    "content_scripts": [
        {
            "matches": ["https://cmywebsite.com"],
            "js": ["content.js"]
        }
    ]
}