Yup, same thing here.
Worked fine with the Beta 6 full installer, but with the Beta 9 full installer, getting the dreaded "unable to create installation media" in Fusion 12 when trying to create a VM.
It's driving me nuts.
We're seeing it too... no workaround just yet.
I'm also finding that when I upgrade a Beta 8 VM to Beta 9, when it's done it falls into a kernel panic loop.
I literally was on the verge of deleting my b6 copy, then tried to install b9 in Fusion, got the error you have and then found this! I successfully extracted the b6 install from my trashcan! :smileygrin:
Following up a bit here, we do see that the disk layout has changed, so we'll need an update to accommodate it unless we can figure a workaround script or something.
I don't suppose my experience will be considered the perfect solution, but it might help someone. I was also faced with the impossibility of creating a new virtual machine using the Big Sur Beta 9 installer, so I simply enrolled a Mojave virtual machine in Apple's beta program and, from within Mojave, I upgraded to Big Sur Beta 9. The only issue is that you need plenty of available space on your virtual disk, or the process will fail. Other than that, Big Sur installed with no issues.