The "Bootable" legacy ISO you've used is in fact not bootable, it relies on CDROM emulation provided by BIOS. VMs don't usually do this sort of emulation. If you set the guest OS to something like ...
The virtual machine may fail to boot from an ISO because the boot order prioritizes the virtual hard disk over the optical drive, resulting in a “No bootable medium ...
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 ...
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