Easy Task, You just need to enable $display_errors = true; on WHMCS config.
WHMCS being broken, commnoly related with simple semicolon or server issue, I can fix it around 1-3 hours, depends what the error is. Let start, and get your site up and running on very short time.
I'm not cheap like others but very reasonable price. No upfront payment is required, I provide 100% satisfaction guarantee. if you don't like my works, you don't need to pay me anything, even a cent. Thanks.