I want to download macOS Sierra from the App Store to create a bootable USB stick (according to Apples web page). However, the App Store tells me that I have installed it and only shows me an "open" button. There is no Install macOS Sierra app in the applications folder. Spotlight however finds it, but it is marked as unknown version and does not start from there. I also cannot find it in Finder. Any ideas?
I see several issues with different solutions. Let's try to fix them one at a time
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.
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.