Automated Orders through WHMCS/Plesk 12 on a Centos7 VPS

Just looking for a bit of assistance with my VPS setup of Centos7/Plesk/WHMCS...

I have successfully integrated WHMCS into my website, and got it linking into the Plesk 12 control panel.

However, when I do a test order through WHMCS I get the error:

Failed to load resource: the server responded with a status of 503 (Service Unavailable)

I then get a configuration error which I think is to do with default VHOST templates etc...:

The subscription/hosting plan IS setup in Plesk, but is not reported back to the WHMCS admin area as completed, so the order remains pending.

Now I'm fairly new to this stuff, but working my way through teaching myself

Any advice would be much appreciated.