Fatal error: Uncaught Error: Class "PKP\mail\variables\Variable" not found in C:\xampp\htdocs\ojs\lib\pkp\classes\mail\variables\QueuedPaymentEmailVariable.php:23 Stack trace: #0 {main} thrown in C:\xampp\htdocs\ojs\lib\pkp\classes\mail\variables\QueuedPaymentEmailVariable.php on line 23