You need to wrap the button inside an tag to make it a clickable link. Also, your CSS has an incorrect @import inside the background-image property—remove that part. Here’s the corrected code:
<html>
<head>
<title>Button Link</title>
<style>
.butn {
background-image: url("https://static1.squarespace.com/static/54da7941e4b0e25dc3648a4f/t/59640885b3db2b282c21c56e/1499728005971/zero_state%40300x-8.png");
width: 555px;
height: 170px;
display: block;
}
.butn:active {
background-image: url('https://static1.squarespace.com/static/54da7941e4b0e25dc3648a4f/t/5964090b6b8f5bf77b28504f/1499728139538/hover_state%40300x-8.png');
}
</style>
</head>
<body>
<a href="https://example.com">
<div class="butn"></div>
</a>
</body>
</html>