Theme hook not found
As seen in webform_example_handler

You need to add:

mymodule/mymodule.module

&lt;?php

/**
 * @file
 * Provides a custom webform handler.
 */

/**
 * Implements hook_theme().
 */
function mymodule_theme() {
  return [
    'webform_handler_myform_form_handler_summary' =&gt; [
      'variables' =&gt; ['settings' =&gt; NULL, 'handler' =&gt; []],
    ],
  ];
}


mymodule/templates/webform-handler-myform-form-handler-summary.html.twig

{#
/**
 * @file
 * Default theme implementation for a summary of an example webform handler.
 *
 * Available variables:
 * - settings: The current configuration for this email handler:
 *   - message: The message.
 *   - debug: Debugging flag.
 * - handler: The handler information, including:
 *   - id: The handler plugin id.
 *   - handler_id: The handler id.
 *   - label: The handler label.
 *   - description: The handler description.
 *
 * @ingroup themeable
 */
#}
{% if settings.debug %}&lt;b class="color-error"&gt;{{ 'Debugging is enabled'|t }}&lt;/b&gt;&lt;br /&gt;{% endif %}
&lt;b&gt;Message:&lt;/b&gt; {{ settings.message }}


Clear/flush the cache so your new hook &amp; template file gets picked up.