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