Virtual machines can seem juuuuust a bit too geeky for most otherwise computer-friendly people. That's sad, because it's an easy, free way to try another operating system, or use those one or two apps ...
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 ...
VirtualBox is great for testing out a new operating system, but your virtual machines probably aren't that special when you first set them up. Here are a few tips for making them much easier to ...