Client area product view error

[Muneef]

when client login to the whmcs panel and click the view details its showing error message.  how can i fix this error.

my current php :8.1 

version 8.6.1

here attached the note pad for error message 

Please help me to fix

Whoops\Exception\ErrorException: Private methods cannot be final as they are never overridden by other classes in /home/demo/clientarea.Demo.com/modules/addons/cWatch/mgLibs/Smarty.php:21
Stack trace:
#0 /home/demo/clientarea.Demo.com/vendor/whmcs/whmcs-foundation/lib/Utility/Error/Run.php(0): WHMCS\Utility\Error\Run->handleError()
#1 [internal function]: WHMCS\Utility\Error\Run->handleShutdown()
#2 {main}
 

Edited December 24, 2022 by Muneef

[WHMCS Anwar]

On 24/12/2022 at 12:36, Muneef said:

Whoops\Exception\ErrorException: Private methods cannot be final as they are never overridden by other classes in /home/demo/clientarea.Demo.com/modules/addons/cWatch/mgLibs/Smarty.php:21

It appears that this error is emanating from a third-party add-on, located in /home/demo/clientarea.Demo.com/modules/addons/cWatch/

Please note: as this is not an add-on supplied or shipped with WHMCS,  you would need to contact the add-on vendor directly for support and updates.

[pRieStaKos]

cWatch addon is not compatible with WHMCS 8.6.x nor PHP8.x, as listed to marketplace.

Remove it or contact the vendor to update the addon.