Creating Catalina bootable installer failed

I followed the directions in HT201372, from a Mojave system, for making a Catalina bootable disk. It ran for most of an hour, incrementing the percentage, but eventually failed. Here is what it says.

Password:

Ready to start.

To continue we need to erase the volume at /Volumes/Untitled.

If you wish to continue type (Y) then press return: Y

Erasing disk: 0%... 10%... 20%... 30%... 100%

Copying to disk: 0%... 10%... 20%... 30%... 40%... 50%... 60%... 70%... 80%... 90%... 100%

Making disk bootable...

Couldn't mount dmg /Volumes/Install macOS Catalina/Install macOS Catalina.app/Contents/SharedSupport/BaseSystem.dmg (error code 112)Mount of BaseSystem failed.

The bless of the installer disk failed.

What does this mean and how to I fix it? It is a clean Mojave.

Thank you.