How to clean install the macOS Catalina?
macOS Catalina runs on a dedicated, read-only system volume called
  Macintosh HD. This volume is completely separate from all other data
  to help prevent the accidental overwriting of critical operating
  system files. Your files and data are stored in another volume named
  Macintosh HD - Data. In the Finder, both volumes appear as Macintosh
  HD.
  https://support.apple.com/en-us/HT210650


Then, these are the procedure.


Click the Macintosh HD - Data (1) then click the minus above the Volume (2)

Click the Macintosh HD (1) then click the Erase (2)



The rest is the same as usual.

Copy right of the images: https://qiita.com/PaSeRi/items/59e9785580dbd518ac93