The install folder needs to be deleted for security reasons before using WHMCS.
WHMCS\Exception\Application\InstallerExists: Application is up to date, but installer directory is still present in /home/manahos/public_html/vendor/whmcs/whmcs-foundation/lib/Utility/Bootstrap/Application.php:0
Stack trace:
#0 /home/manahos/public_html/init.php(0): WHMCS\Utility\Bootstrap\Application::verifyInstallerIsAbsent()
#1 /home/manahos/public_html/cart.php(0): unknown()
#2 {main}