I find that every now and again VirtualBox glitches out and fails to install (or update) on macOS. This has happened to me a few times now, and every single time it's because Apple security policies prevent VirtualBox kernel extensions from being loaded.
Attempt 1 of 2: Start with OSXDaily fix: check your "System Preferences > Security & Privacy > General". There might be a discreet note in the lower part of the window saying that kernel extensions from the developer Oracle could not be loaded and an option to whitelist them by clicking on the "Allow" button. Done! :)