This problem really annoys me - and it seems like either a Windows bug or poor design.
Hopefully it's neither and it's just me :)
I have reproduced it in Windows Server 2016/2019 Datacenter
1) Create a Virtual Machine, install Windows Server Datacenter
2) Add multiple drives to the VM
3) Add multiple drives to a Storage Pool (make sure that you set MediaType to both HDD and SSD)
4) Try to create a Virtual Disk through the GUI (actually it fails in Powershell aswell). Make sure you set the Size to Maximum size.
This will fail and you will get an error saying you don't have enough resources but in fact you can just choose a lower size and it will work just fine. I tried to play around with the sizes and I think that the only problem here is that the "Maximum size" is calculated wrong thus given an error.