script tagınızı (<script src="java.js"></script>)
</body> bitmeden önce ekler misiniz?

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <link rel="stylesheet" href="style.css">
 
  <title>Document</title>
</head>
<body>
 
  <div class="container">
    <p class="heading">Sayaç</p>
    <p class="counter" id="counter">0</p>
    <button class="incr">+</button>
    <button class="decr">-</button>
  </div>
  <script src="java.js"></script>
</body>
</html>