It is the first thing you need to fix the issue as stated above. For that, you need to run the virtual machine showing the error and go to the Devices > Install Guest ...

If you cannot open the guest session and get a VERR_NOT_FOUND VirtualBox error, follow the tips, solutions, and workarounds mentioned below. Check if “VBoxGuestAdditions.iso”n exists Insert Guest ...

VirtualBox is my go-to virtual machine technology. Not only is it free to use, but it’s also very powerful and makes it easy to create VMs for all types of hosts — from Linux, macOS, Windows and more.