@evansims Thanks for your suggestion.
I have removed session_start() and added suggested SessionStore code.
Now I’m getting new error
[Tue Jul 23 17:52:23.946676 2024] [php:error] [pid 1802892] [client 192.168.81.227:55173] PHP Fatal error: Declaration of GuzzleHttp\\Psr7\\Request::getRequestTarget() must be compatible with Psr\\Http\\Message\\RequestInterface::getRequestTarget(): string in /var/www/html/whmcs/vendor/guzzlehttp/psr7/src/Request.php on line 58
[Tue Jul 23 17:52:23.963816 2024] [php:notice] [pid 1802892] [client 192.168.81.227:55173] [WHMCS Application] ERROR: Whoops\\Exception\\ErrorException: Declaration of GuzzleHttp\\Psr7\\Request::getRequestTarget() must be compatible with Psr\\Http\\Message\\RequestInterface::getRequestTarget(): string in /var/www/html/whmcs/vendor/guzzlehttp/psr7/src/Request.php:58 Stack trace: #0 /var/www/html/whmcs/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} {"exception":"[object] (Whoops\\\\Exception\\\\ErrorException(code: 64): Declaration of GuzzleHttp\\\\Psr7\\\\Request::getRequestTarget() must be compatible with Psr\\\\Http\\\\Message\\\\RequestInterface::getRequestTarget(): string at /var/www/html/whmcs/vendor/guzzlehttp/psr7/src/Request.php:58)"} []