// Решение:
let amountArmy = 0;
function trainArmy() {
amountArmy = amountArmy + 1;
console.log(amountArmy);
}
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<link rel="stylesheet" href="style.css">
</head>`введите сюда код`
<body>
<div class="main">
<div class="amount">
<h1 class="amount-army" id="amount-army">Количество армии:</h1>
<h2 class="amount-power" id="amount-power">Мощь:</h2>
</div>
<div class="train">
<button onclick="trainArmy()">Click</button>
</div>
</div>
<script src="app.js"></script>
</body>
</html>