Hi there!
I installed VirtualBox step by step as detailed here: https://getsol.us/articles/software/virtualbox/en/ .
Created 50GB space for a new OS (wanted to try out Fedora, didn't work, then openSUSE, didn't work either), went through the settings for the appropriate virtual OSs and don't know how to continue:
VirtualBox keeps saying that it cannot find a bootable device although during the installation process it was said that using a USB stick should be possible.
I'm running Budgie under the 5.1.14-121.current kernel, updated everything before installing VirtualBox, installed extension pack 6.0.8 as detailed here https://www.virtualbox.org/wiki/Downloads, selected the USB controller for USB 3.0 (as I want to use USB 3.0 with my Thinkpad W530 laptop) - no success.

Any idea?
Do you need more information?

SOLUSfiddler

From what I'm able to tell from a quick search USB 3.0 support isn't as mature in VirtualBox as USB 2.0/1.1, it probably supports it as a USB device in itself but try switching the USB mode just for the installation process and see what happens (and then switch it back after you've finished installation).

Not actually replicated this myself since I don't even have a USB 3.0 port on my desktop, but I'd take a shot at that. Installation will obviously be slower than if it worked in the first place but it would probably work fine afterwards...