I just purchased a license for WHMCS and went ahead to install using the installer and I'm getting this:
Found http://software.virtualmin.com:80/download/whmcs_v5215.zip in cache .. Found http://downloads2.ioncube.com:80/loader_downloads/ioncube_loaders_lin_x8... in cache ..
Configuring webserver PHP settings .. .. already done. Applying web server configuration .. .. done
Now installing WHMCS version 5.2.15 .. System check failed More information on using this script can be found at http://www.whmcs.com/members/aff.php?aff=4115.
.. installation was only partially complete. Re-loading Webmin .. .. done
Applying web server configuration .. .. done
I poked around but can't see why it failed. Nothing obvious in the logs.