Mojave never was an IPSW, because it never ran on Apple Silicon. I imagine this tool might just not support that at all.

LoganDark is right. I've personally never tried, and don't think it'd be easy for any macOS predating Apple Virtualization Framework. For that you'd need something like UTM since they're relying on QEMU - these configs might help: https://github.com/adespoton/utmconfigs

Ahh I see. UTM was what I was trying, so I’ll give those a look! Thanks