fori,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style>
body{
margin: 0;
overflow: hidden;
display: flex;
}
.wrapper{
display: flex;
width: 100vw;
height: 100vh;
justify-content: center;
flex-direction: column;
}
.Rectangle{
width: 100vw;
height: 20vh;
background: green;
}
.But{
width: 100vw;
text-align: center;
font-size: 10vmin;
}
</style>
</head>
<body>
<div class="wrapper">
<div class ="Rectangle"></div>
<div class="But">Выйти</div>
</div>
</body>
</html>