How to display a button with “markup”?
You can use the inline_template to solve the problem:

// Delete the other button.
unset($form);
$form['submit'] = [
  '#type' =&gt; 'inline_template',
  '#template' =&gt; '&lt;button type="button" class="btn btn-success" disabled="disabled"&gt;Product unavailable&lt;/button&gt;',
];