Alpha SSL, Cloudflare, WP Rocket, Hummingbird & WHMCS

Hello. Hope everyone in a good mood at weekend to help resolved my setup.
I am trying to install WHMCS in gagabyteswebsolutions.com which has Alpha SSL, Cloudflare connected, WP Rocket and Hummingbird plugin. Full SSL is working but when I try to install WHMCS in root with folder via Softaculous and choose https -- I am getting an error "A trusted SSL Certificate was not found". Is someone has gotten the same issue before? And how did it resolved the issue?

Many Thanks,

Jo