Mac – How to download macOS High Sierra from the Mojave App Store

I see several issues with different solutions. Let's try to fix them one at a time

Spotlight

First you should check if your Spotlight index is current. Search again for the Sierra Installation App. If it is listed, scroll down that list and click on "Show all in finder" (see Apple Support Page Description).

You get a Finder Window with all the Search results and if the Installer App is listed there you can get the Location and try to create the bootable USB Stick from there. You should also check the File Size and the Permissions of the Installer in case it is f.e. File Size 0 or you don't have permission to execute the Installer. You could also delete that Installer in order to reload it from the App Store.

If the Installer is showing in Spotlight, but is not showing up in the Finder you should rebuild your Spotlight Index (see Apple Support Page here or use a third party tool like Onyx, but be carful). Depending on your HD Size and Speed that could take some hours.

App Store

If the App store is still showing that the installer is present and won't let you download, you could try to delete (or just move it somewhere) your App Store Cache at ~/Library/Caches/com.apple.appstore. You should restart at that point and then try to Download from the App Store again.

There is a second (temporary) cache Location but only for downloads. You can get there fastest by opening the Terminal and typing in open $TMPDIR../C/. There you will find a Folder named com.apple.appstore which you could also move, restart, then try to DL again.

Try these approaches first and see if that solves your Problem.

You will need to delete the current installer. OS X applications are usually very good about keeping track of files, even when they get moved (see How does Mac OS keep track of moved files?). In this case, the AppStore is likely keeping track of the application bundle (which you would normally want it to do) when you move it.