Configure Checkout Page, WHMCS and cPanel for automation of Web/VPS Hosting

Had a website built, had my ProxMox server configured with WHMCS for billing and automation, and cPanel for hosting clients with softalicious (All licenses purchased and valid). Everything seems to work great, but checkout does not complete. It goes to a blank white page. In WHMCS, there is an account created with user/pass, but the user receives no email notification. When I manually send the user the password, cPanel advises the user that the account is not valid. So while things are allocated and created, absolutely NOTHING works, and the client is never notified. I need this fixed as absolutely quickly as possible. The company that built the site and configured it does not seem to be able to get it working, and I am loosing users. I intend to ask the previous developer for a partial refund as I am pleased with the website and the software as installed, but none of it works. Place your bids, and please, if you bid, be willing to commit to get it done fast. Again, the site is NGIX. The server is running Proxmox. WHMCS is running in a virtual machine on the server. WHMCS, cPanel etc are all licensed, installed and ready. They are allocated to the server IP and ready to go, they just need to be tied together correctly so that they function.