As stated on this page: https://docs.whmcs.com/Plesk#Plesk_OnyxPlesk 17.8 makes a number of backwards incompatible changes to the API. As such the module for WHMCS is not currently fully compatible. We suggest operating Plesk 17.5 at this time.
Plesk Onyx 17.8 has been released for quite some time now, in fact, 17.9 is not too far away.
Please implement full support for Plesk 17.8 / 17.9 via the built-in WHMCS Plesk module.
It would be good to know what features are "not currently fully compatible" with 17.8.