Please read chapter 5.1 of the User Manual. Booting from USB media requires adding them to a virtual USB controller. Additionally, this works only if UEFI is selected, the BIOS implementation lacks any USB boot support.
Thank you again... things like that are the reason why everybody is such a fan of Linux. I have in front of me a book called Linux for beginners, from bdm-publications 2020, where I find perfect instructions on how to create the bootable USB stick. Given that hardly any laptop nowadays still has a DVD player, that is great step froward. It even specifically points out that the available correct extension for Virtual Box is necessary... And of course there is no mention of UEFI/Bios This must have been my tenth attempt to test Linux since open suse 15 years ago, the only one that actually worked smoothly. Could it be that this one of the major reasons why Linux never really takes off? Thank you, at least you are saving m some time. Win7 support runs another two and a half years. Maybe then I try again...