_marisha,
что не так?
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">.row__letter {
background-color: #FF0000;
height: 5px;
}
</style>
<script>
document.addEventListener( "DOMContentLoaded" , function() {
let board = document.querySelector('.board');
let rowOne = document.createElement('div');
rowOne.className = 'row__letter';
board.append(rowOne);
let rowEnd = document.createElement('div');
rowEnd.className = 'row__letter';
board.prepend(rowEnd);
});
</script>
</head>
<body>
<div class="board">
<div class="row" id="0">
<div class="cell">0</div>
<div class="cell">0</div>
</div>
<div class="row" id="1">
<div class="cell">1</div>
<div class="cell">1</div>
</div>
</div>
</body>
</html>