Creating Sierra in OSCatalina with Parallels Desktop 16

I have been exploring ways of creating Sierra 10.12 in Parallels Desktop 16 for Mac. I tried downloading the .dmg and .iso files but Parallels did not recognize the files. I was able to install Ubuntu 20.04 and Windows 10. Unfortunately, neither the Sierra nor El Capitan OS was not recognized by Parallels 16. I need these OS because my software can work run there.
Help on doing this will be appreciated.

 

Hello,
Are you able to see the download OS files when you click on "Choose manually"?