You cannot apply the Combo to the installer you already have.
You would need to install macOS Catalina 10.15.1 and then install the Combo update over that.
Or just ditch the installer you already have and download the last version of the installer from the App Store.
Open Safari and click on this link,
How to get old versions of macOS – Apple Support (UK)
Go to the Download OS section and click on macOS Catalina.
This will redirect you to the App Store Catalina download page,
it should say Get or Download.
This will download the last version of Catalina 10.15.7 directly to your Applications folder
as Install macOS Catalina.app, it should be 12.26GBs in size.
Make a bootable USB installer with that.