Custom module administration page template not working
It is a well known issue of the hook_theme() function.

For successfully loading the template file, you have to follow the naming convention to rename the template name, which must be same as the theme key, with converting _ to -.

/**
 * Implements hook_theme()
 */
function module_custom_theme($existing, $type, $theme, $path) {

    $template_path = drupal_get_path('module', 'module_custom') . '/templates';

    return array(
        'user_availability' =&gt; array(
            'variables' =&gt; array('users', NULL),
            'template' =&gt; 'user-availability',
            'path' =&gt; $template_path
        )
    );

}