How download installer for macOS

To perform this action will require booting from a Bootable Installer. 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer.

Example : Bootable Installer of Big Sur would have to be done on a computer that Qualifies to run Big Sur.

Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can not be used.

How to get old versions of macOS - Apple Support

You cannot download a version of macOS that is older than what was shipped with the Mac you are using. Which Mac model & year are you using to download and which version of macOS are you trying to download?

You didn't state which Mac/model/year.