Actually, this is not the documented bug mentioned by @SirCharlo. In fact, it's not really a bug at all. Startup Disk Creator is designed to only make Ubuntu startup disks. If you read the description of the program, it does actually say this, as I found out after the fact.
In my case, I was trying to make a Debian startup disk, and it has an OS Version labled "Debian GNU/Linux...", and therefore the app will not use it to make the disk. Making a Ubuntu disk works perfectly.
It would be nice if it gave a better error message, but the behavior is evidently by design.