Which process did you use to create the installation media?
If you click to open the diagnostics log file shown there, does it open? If so, you may be able to connect an additional USB flash disk and save the log there, then you’ll be able to share it here.
If you are able to use the USB stick to get to a live session, rather than running the reformatter directly from initial setup, it may be easier to save the log elsewhere. You also may be able to use GNOME Disks to unmount the partition and then try the reformatter again. (Even if that works I’d be interested in the log!)
Thanks for providing the log. That’s a weird failure, we’ll have to look into what’s going on there.
However the following workaround should work to get you started:
Boot up but choose to Try Endless rather than reformatting
Open Terminal
In terminal run sudo umount /dev/sdc2
Open Reformat app from desktop icon
Continue the installation
Step 3 can alternatively be done within the Disks if you find your USB disk there, click on the 2nd partition and then click on the “stop” button to unmount it.