Creating MacOS Mojave recovery disk (2013 Macbook Air) on my MacOS Big Sur laptop (2018 Macbook Pro)

Trying to create the bootdisk, but after running the terminal script, and the usb drive is erased and renamed "Install MacOS Mojave", the drive won't mount. How can I fix this?

Password:
Ready to start.
To continue we need to erase the volume at /Volumes/MyVolume.
If you wish to continue type (Y) then press return: y
Erasing disk: 0%... 10%... 20%... 30%... 100%
The erased disk did not mount.

The laptop I'm creating the bootdisk from is a 2018 Macbook Pro (running Big Sur), and I'm trying to recover my 2013 macbook air (to MacOS Mojave).

An underlying issue that is likely causing this failure is that the USB drive will unmount whenever it is renamed/erased, and then will not mount again until I log out of the deskttop and log in again. Disk Utility shows this error when trying to mount the drive:

Could not mount “Install macOS Mojave”. (com.apple.DiskManagement.disenter error -119930872.)