VirtualBox isn't, and has never been, a supported hypervisor.
Depending on your VM-Series model you can run the VM on ESXi, KVM, Hyper-V, AWS, AZURE, NSX, or SDX. You can find more information about which hypervisor will run which VM in the documentation on this PAGE.
The only sucessful install on virtualbox I've ever seen was when they built a 'lab in a box' type of install with a VM-Series and GNS3 running on the local host; but that takes a lot of playing and I haven't seen that with a current PAN-OS version at all, so I'm not sure it would actually still function.
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