Clean install of Catalina

I'm trying to do a clean install of Catalina on my mid-2014 MacBook Pro that I want to give to someone else. I went into disk utility and erased the "Macintosh HD - Data" volume. I also attempted to erase the "Macintosh HD" volume but got an error saying the volume was in use. I then rebooted using Command-R and tried erasing the "Macintosh HD" again but got the same error. Next, I selected "Reinstall macOS" from the macOS Utilities menu. I was asked to select a location for installing the operating system. The options were "Macintosh HD - Data" and "Macintosh HD". My understanding was that the OS should be installed on the "Macintosh HD" volume, however when I selected it I got an error saying that the OS could not be installed at that location. It would only allow me to install the OS on the "Macintosh HD - Data" volume.

So, obviously I'm doing something wrong. Could someone tell me what I need to do to clean the drive of all my personal data and then do a clean install of Catalina so that it will boot up to the "Hello" screen and allow the new owner to set it up for themselves.

TIA.