O códigos HTML que estão no curso estão com a estrutura errada, vejam o código abaixo que está em "04 - Formulário de novos produtos"
{{define "New"}}
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T"
crossorigin="anonymous">
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js" integrity="sha384-JjSmVgyd0p3pXB1rRibZUAYoIIy6OrQ6VrjIEaFf/nJGzIxFDsf4x0xIM+B07jRM"
crossorigin="anonymous"></script>
<title>Alura loja</title>
</head>
<nav class="navbar navbar-light bg-light mb-4">
<a class="navbar-brand" href="/">Alura Loja</a>
</nav>
<div class="container">
<body>
<div class="jumbotron jumbotron-fluid">
<div class="container">
<h1 class="display-5">Novo produto</h1>
<p class="lead">Insira os detalhes</p>
</div>
</div>
<form method="POST" action="insert">
<div class="row">
<div class="col-sm-8">
<div class="form-group">
<label for="nome">Nome:</label>
<input type="text" name="nome" class="form-control">
</div>
</div>
</div>
<div class="row">
<div class="col-sm-8">
<div class="form-group">
<label for="descricao">Descrição:</label>
<input type="text" name="descricao" class="form-control">
</div>
</div>
</div>
<div class="row">
<div class="col-sm-2">
<div class="form-group">
<label for="preco">Preço:</label>
<input type="number" name="preco" class="form-control" step="0.01">
</div>
</div>
</div>
<div class="row">
<div class="col-sm-2">
<div class="form-group">
<label for="quantidade">Quantidade:</label>
<input type="number" name="quantidade" class="form-control">
</div>
</div>
</div>
<button type="submit" value="salvar" class="btn btn-success">Salvar</button>
<a class="btn btn-info" href="/">Voltar</a>
</form>
</body>
</html>
</div>
</body>
</html>
{{end}}