Unable to update to Mojave
Having come across a similar error myself, I found that it won't install Mojave onto an external hard disk due its need for the EFI partition to be on a local disk. The EFI partition is only 200MB+, so in theory you could get away with installing a very small hard-drive internally and just stick the EFI partition there. Once Mojave's installed, you can move the EFI partition to a USB (probably needs to be placed before any other partitions).

I use Linux/Gparted to change partitions for this purpose. You can do that entirely from e.g. the Ubuntu install USB without having to install Linux at all. You probably have an EFI partition somewhere on your USB drive, so simply copy it to an internal hard-drive. Since it's probably only 250MB, you can get such drives for extremely cheap off eBay if you don't have a drive lying around somewhere from an archaic machine.