If you don't first explicitly format the virtual disk, the macOS installer will not find any disks to install macOS on.
Erase/format it as Mac OS Extended Journaled.
In Disk Utility, the hard disk will show up as a disk on the left. Erase), the virtual disk you created during the initial creation of the VirtualBox Guest VM. When the recovery partition's GUI comes up (wherein you have the option to reinstall (also used for initial install)), first choose Disk Utility, and find and format (e.g.