This happens when you run your system on stable branch.
If you don’t have specific needs like the ones mention in above guide - you can safely live without the extension pack.
If you need if for USB passtrrough - it work much better to mount the usb devices in a predictable structure and share that structure to the virtual machine.
Best practise on sync is
- sync your syetem - reboot if important system packages has been synced
- rebuild your custom packages