McAfee's policy has always been not to support test or beta environments and 8.1 is still in the testing phase.
There is a release at McAfee Beta (Consumer software) which will support it and I quote:
New machines (new registrants) who register at beta.mcafee.com will have their machines ‘sniffed’ for OS version.
If OS = Windows 8.1, they will automatically get 12.8.1 (and no other beta version).
Existing machines, non-Win8.1, who already have 12.8 installed will still get updated to 12.8.1 even though their OS may not be Windows 8.1
This is because 12.8.1 can work on the other platforms as well as Win 8.1.
In order to obtain beta software you must register a new online account at beta.mcafee.com using a different email than the one used at your regular online account and details of how to join and where support can be obtained for beta products is here: https://community.mcafee.com/docs/DOC-1236
(No support in the regular forums).