Class "JFormHelper" is no longer supported in J5 with the J5 Compatibility Plugin disabled. Therefore, you definitely need the J5-native version of the template. Regarding the current support situation at Joomla51 (which is far from perfect, to describe the situation politely) it will be difficult for you to get reach of that version.

Also, even the J5 version will need the J6 Compatibility plugin to be functional under J6.

Have you enabled all settings in the backward compatibility plugin?

Thank for your remarks. The Backwards Compatibility Plugin is enabled. I use the latest version of Layla that show in My Downloads a Layla 4. There is no separate Layla 5. On the J51 it is mentioned as being compatible with Joomla 4,5,6.
These are the errors that I receive after making Layla the default template:

An error has occurred.

0 Class "JFormHelper" not found

Call Stack
# Function Location
1 () JROOT/templates/j51_layla/elements/tplhelper.php:16
2 require_once() JROOT/libraries/src/Form/FormHelper.php:271
3 Joomla\CMS\Form\FormHelper::loadClass() JROOT/libraries/src/Form/FormHelper.php:136
4 Joomla\CMS\Form\FormHelper::loadType() JROOT/libraries/src/Form/FormHelper.php:78
5 Joomla\CMS\Form\FormHelper::loadFieldType() JROOT/libraries/src/Form/Form.php:1463
6 Joomla\CMS\Form\Form->loadField() JROOT/libraries/src/Form/Form.php:304
7 Joomla\CMS\Form\Form->getFieldset() JROOT/layouts/joomla/edit/fieldset.php:19
8 include() JROOT/libraries/src/Layout/FileLayout.php:128
9 Joomla\CMS\Layout\FileLayout->render() JROOT/libraries/src/Layout/LayoutHelper.php:76
10 Joomla\CMS\Layout\LayoutHelper::render() JROOT/layouts/joomla/edit/params.php:170
11 include() JROOT/libraries/src/Layout/FileLayout.php:128
12 Joomla\CMS\Layout\FileLayout->render() JROOT/libraries/src/Layout/LayoutHelper.php:76
13 Joomla\CMS\Layout\LayoutHelper::render() JROOT/administrator/components/com_templates/tmpl/style/edit.php:115
14 include() JROOT/libraries/src/MVC/View/HtmlView.php:416
15 Joomla\CMS\MVC\View\HtmlView->loadTemplate() JROOT/libraries/src/MVC/View/HtmlView.php:204
16 Joomla\CMS\MVC\View\HtmlView->display() JROOT/administrator/components/com_templates/src/View/Style/HtmlView.php:97
17 Joomla\Component\Templates\Administrator\View\Style\HtmlView->display() JROOT/libraries/src/MVC/Controller/BaseController.php:697
18 Joomla\CMS\MVC\Controller\BaseController->display() JROOT/administrator/components/com_templates/src/Controller/DisplayController.php:68
19 Joomla\Component\Templates\Administrator\Controller\DisplayController->display() JROOT/libraries/src/MVC/Controller/BaseController.php:730
20 Joomla\CMS\MVC\Controller\BaseController->execute() JROOT/libraries/src/Dispatcher/ComponentDispatcher.php:143
21 Joomla\CMS\Dispatcher\ComponentDispatcher->dispatch() JROOT/libraries/src/Component/ComponentHelper.php:361
22 Joomla\CMS\Component\ComponentHelper::renderComponent() JROOT/libraries/src/Application/AdministratorApplication.php:150
23 Joomla\CMS\Application\AdministratorApplication->dispatch() JROOT/libraries/src/Application/AdministratorApplication.php:206
24 Joomla\CMS\Application\AdministratorApplication->doExecute() JROOT/libraries/src/Application/CMSApplication.php:320
25 Joomla\CMS\Application\CMSApplication->execute() JROOT/administrator/includes/app.php:58
26 require_once() JROOT/administrator/index.php:32

You need the J5 version of the template And you can run this under J6 only if the J6 Compatibility Plugin is enabled.

Is the Joomla plugin Behaviour - Backward Compatibility enabled?

Can you send us a link to the page with the error message enabled?

Hello,
Even with that plugin enabled, I cannot get the J51 4 versions to work in Joomla 6.2. This is what I get:
The error
"0 Class 'JFormHelper' not found" And AI tells me : this is a common critical error in Joomla, typically occurring when a site is updated to Joomla 4 or Joomla 5.
When I dive deeper in this problem, I get pages full of codes that I do not understand.
What is the solution?
Kind regards, Wouter

Download the Joomla 4 version and install it in Joomla 6. The backward compatibility plugin must be enabled.