Hello,
The same issue has also been reported at the VMware Fusion forum and while the product is different, the problem is the same.
Will I be able to reserve a copy of Windows 10 using my Windows 8.1 VM?
It is however not a problem to install and run Windows 10 under VMware, it is the "get Windows 10" program that complains.
Some nice steps on how-to install/update to Windows 10 are available at:
--
Wil