What OS did you install? Big Sur or something older? If you have an older OS, then just download & install Catalina using the link within the Apple article I provided previously since it includes a link to an Apple article with instructions for downloading other macOS installers.
However, if you installed Big Sur and you need Catalina, then you need to perform another clean install since Catalina is immediately before Big Sur. You can use the same instructions that you followed previously for creating a bootable macOS USB installer. That article links to another Apple article that has links for various macOS installers.