Bootstrap 4 dropdown not working properly

Question

Learning Bootstrap 4 and the following are the simple codes. The dropdown is not dropping down or working. When I tried with the select it's working. What are the ways to do things right?

echo '<div class="dropdown">';
echo '<button class="btn btn-primary dropdown-toggle" type="button" data-toggle="dropdown">'.strtoupper($field).'</button>';
           echo '<div class="dropdown-menu">'; 
           for($i = date('Y'); $i >= 1900; $i--) {
                  echo "<a class='dropdown-item' href='#'>$i</a>";
           }
echo '</div></div>';

Answer 1

Try this:

   echo "<ul>";
  for($i = date('Y'); $i >= 1900; $i--) {
     echo "<li><a class='dropdown-item' href='#'>$i</a></li>";
  }
  echo "</ul>";