Bootable drive - disk is locked error
You must select a drive that’s not the installer in step 7 since the volume where the installer exists is read only until the install completes. 

I’ll assume this is your Apple guide. 


https://support.apple.com/en-us/HT204904


Any drive except the installer you prepared should be chosen. This lock is to force you to  it over write the installer. 


if you want to erase the main drive - https://support.apple.com/en-us/HT208496
If you have room, make a second volume on APFS beside the Catalina so you can put Mojave not on your main drive and not on the main external partition. 


This is something that trips up people that come from Knowing how to install other OS. Use internet recovery if you have no bootable macOS to run an installer rather than copying your installer onto the volume you want actual macOS on. 

Also, since you may not wish to erase your Mac (it’s running Catalina and you want Mojave on an external drive) and if there’s not enough free space on the internal or external for a second volume to hold Mojave, get a third external drive so the installer can be on one external and boot to it to install to the second external drive (or second volume).