Трябва да ги кажа, че доста сложен код,

опростих го малко, направих промени към по-стар JS и то тръгна.

Не съм сигурен, но е проблем moethod append, вместо appendChild

function solve() {

  // let inputField = document.getElementsByTagName('textarea')[0];

  document
    .getElementsByTagName('button')[0]
    .addEventListener("click", generate);
  // generateButton.addEventListener("click", generate);

  document
    .getElementsByTagName('button')[1]
    .addEventListener("click", buy);
  // buyButton.addEventListener("click", buy);

  function generate(e) {

    let objects = JSON.parse(
      document
        .getElementsByTagName('textarea')[0]
        .value
    );
    // objects.forEach(obj => objectGenerator(obj))
    objects.forEach(function (obj) { objectGenerator(obj) })

  }

  function objectGenerator(obj) {
    // let name = obj.name;
    // let img = obj.img;
    // let price = obj.price;
    // let decFactor = obj.decFactor;

    // let tbodyElement = document.getElementsByTagName('tbody')[0];

    let trElement = document.createElement('tr');
    let imgElementTd = document.createElement('td');
    // let imgElement = document.createElement('img');
    // imgElement.src = img;
    imgElementTd.innerHTML = '<img src="' + obj.img + '">';

    document
      .getElementsByTagName('tbody')[0]
      .appendChild(trElement);

    // tbodyElement.appendChild(trElement);
    // trElement.append(imgElementTd);
    trElement.appendChild(imgElementTd);

    let nameElementTd = document.createElement('td');
    let nameElementP = document.createElement('p');


    nameElementP.textContent = obj.name;
    nameElementTd.appendChild(nameElementP);
    trElement.appendChild(nameElementTd);


    let priceElementP = document.createElement('p');
    priceElementP.textContent = obj.price;

    let priceElementTd = document.createElement('td');
    priceElementTd.appendChild(priceElementP);

    trElement.appendChild(priceElementTd);

    let decFactorElement = document.createElement('td');
    let decFactorElementP = document.createElement('p');

    decFactorElementP.textContent = obj.decFactor;
    // decFactorElement.append(decFactorElementP);
    decFactorElement.appendChild(decFactorElementP);
    trElement.appendChild(decFactorElement);

    // document
    //   .createElement('input')
    //   .type = 'checkbox';
    // checkBox.type = 'checkbox';
    let checkBoxTd = document.createElement('td');
    checkBoxTd.innerHTML = '<input type="checkbox">';
    // trElement.append(checkBoxTd);
    trElement.appendChild(checkBoxTd);

  }

  function buy(e) {
    let boughtFurniture = [];
    let totalPrice = 0;
    let avgDecFactor = 0;
    let count = 0;

    let allTr = Array.from(document.getElementsByTagName('tr'));
    // console.log(allTr[2].children[4].children[0]);
    for (let i = 2; i < allTr.length; i++) {

      if (allTr[i].children[4].children[0].checked) {
        count += 1;
        boughtFurniture.push(allTr[i].children[1].textContent);
        totalPrice += +allTr[i].children[2].textContent;
        avgDecFactor += +allTr[i].children[3].textContent;

      }
      // console.log(boughtFurniture);
    }

    avgDecFactor = avgDecFactor / (count);
    // avgDecFactor = (avgDecFactor / (allTr.length - 2)).toFixed(2);

    document
      .getElementsByTagName('textarea')[1]
      .value =
      "Bought furniture: " +
      boughtFurniture.join(', ') +
      "\nTotal price: " +
      totalPrice.toFixed(2) +
      "\nAverage decoration factor: " +
      avgDecFactor;
  }

}