Problem comes from result variable in your buy function, check out and compare zero test results for your code :

Unexpected error: Output textarea is different: expected 'Bought furniture: 
Tablet, Vase\n    Total price: 2015.00\n    Average decoration factor: 7.6' to equal 
'Bought furniture: Tablet, Vase\nTotal price: 2015.00\nAverage decoration factor: 7.6'

Change:

 const result = `Bought furniture: ${names.join(', ')}
    Total price: ${total.toFixed(2)}
    Average decoration factor: ${decFactor / furniture.length}

to:

    const result = `Bought furniture: ${names.join(', ')}\nTotal price: ${total.toFixed(2)}\nAverage decoration factor: ${decFactor / furniture.length}`;

Recommended reading on this exercise => https://softuni.bg/forum/30365/08-furniture-js-advanced-dom-exercise

Alternative code with similar structure:

function solve() {
    let generateBtn = document.querySelector('button');
    let buyBtn = Array.from(document.querySelectorAll('button'))[1];
 
    const tableBody = document.querySelector('tbody');
    generateBtn.addEventListener('click', generate);
    buyBtn.addEventListener('click', buy);
 
 
    function generate() {
        let textArea = document.querySelector('textarea').value;
 
        let parsedArray = JSON.parse(textArea);
 
        for (let furniture of parsedArray) {
            let tableRowElement = document.createElement('tr');
 
            let tdImage = document.createElement('td');
            let tdName = document.createElement('td');
            let tdPrice = document.createElement('td');
            let tdFactor = document.createElement('td');
            let tdMark = document.createElement('td');
 
            tableRowElement.appendChild(tdImage);
            tableRowElement.appendChild(tdName);
            tableRowElement.appendChild(tdPrice);
            tableRowElement.appendChild(tdFactor);
            tableRowElement.appendChild(tdMark);
 
            let image = document.createElement('img');
            image.src = furniture.img;
            tdImage.appendChild(image);
 
            tdName.innerText = furniture.name;
            tdPrice.innerText = furniture.price;
            tdFactor.innerText = furniture.decFactor;
 
            let checkBox = document.createElement('input');
            checkBox.type = 'checkbox';
 
            tdMark.appendChild(checkBox);
 
            tableBody.appendChild(tableRowElement);
        }
    }
 
    function buy() {
        let resultTextArea = Array.from(document.querySelectorAll('textarea'))[1];
        let output = [];
 
        let toBuy = Array.from(document.querySelectorAll('input[type="checkbox"]'))
            .filter(e => e.checked === true)
            .map(e => e.parentNode.parentNode);
 
        let bought = toBuy.map(e => Array.from(e.querySelectorAll('td'))[1].innerText)
            .join(', ');
 
        output.push(`Bought furniture: ${bought}`);
 
        let totalPrice = toBuy.map(e => Array.from(e.querySelectorAll('td'))[2].innerText).map(Number)
            .reduce((acc, current) => acc + current, 0);
 
        output.push(`Total price: ${totalPrice.toFixed(2)}`);
 
        let averageFactor = toBuy.map(e => Array.from(e.querySelectorAll('td'))[3].innerText).map(Number)
            .reduce((acc, current) => acc + current, 0) / toBuy.length;
 
        output.push(`Average decoration factor: ${averageFactor}`);
 
        resultTextArea.value = output.join('\n');
    }
}