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.
MacBook Pro 15", macOS 10.15