Hi,
i do not have my Librem 5 already. So i have no experience with the Flash Mode.
But without the uuu Package on your Debian (in a VM?) i would suggest to set up a PureOS Live-USB Stick. Boot your Notebook or Desktop via Live-USB from your USB-Stick and run the Reflashing Firmware Process from there.
https://tracker.pureos.net/w/recommendations/#preparing-to-install
Take Care, you will lose all your Data on that USB-Stick you will using for a Live-USB. So have a Backup if you need one.
OK, you installed uuu already but still use a VM?
Did you noticed on the Top Left of the Picture the Issue about “Failed to attache the USB Device” … “to the Virtual Machine” …?
If you can and now, how to boot from a Liveusb, i would suggest this way to https://forums.puri.sm/t/new-post-reflashing-the-librem-5/11867
Good Luck and hope i did not miss something important.