Flash drive isn't mounting correctly, this also results in missing drivers/firmware, try again recreting the flash drive on the same or a different one if possible, manually creating might also help.
In Windows open a command prompt window as administrator, then type, in this order:
-diskpart
-list disk
-select disk x (x=your flash drive)
-clean
-create partition primary
-format fs=fat32 label=UNRAID quick
-active
-assign
-exit
-close cmd window
-extract Unraid release to flash
-execute make_bootable as admin