.main {
display: flex;
align-items: center;
justify-content: center;
}
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<link rel="stylesheet" href="style.css">
<link rel="preconnect" href="https://fonts.googleapis.com">
<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
<link href="https://fonts.googleapis.com/css2?family=Kaushan+Script&family=Montserrat:ital,wght@0,400;0,500;0,600;0,700;0,800;0,900;1,400;1,500;1,600;1,700;1,800;1,900&display=swap" rel="stylesheet">
<title>Тест на IQ</title>
</head>
<body>
<div class="main">
Приветствую
</div>
</body>
</html>
Не работает align-items: center