I faced the same problem.
The issue happened on Ubuntu22. The problem which I faced is that the disks software format the entire flashdrive in a single partition.
The partition was like that:
UNRAID -> ../../sdd
The issue is that the script make_bootable_linux expects the drive to be with a number at the end.
I reformat the flashdrive with gparted. Made a new partition table and created a second partition a few MB (not sure if this is necessary)
Now my UNRAID partition is sdd1 and the the bootable disk is ready.