Software update 10.13.6 doesn't show up on iMac Sierra Mid 2011 app store

I'm trying to edit my videos on Final Cut Pro X, but when I try to download it, it says I need to be updated to version 10.13.6. So I go to update to that version, but the only software update available is version 10.12.6. So I update to that, thinking version 10.13.6 would show up after that, but it hasn't. I've tried everything, (restarting computer, reloading app store updates, etc.) and NOTHING works. I try to download it from the apple website, but when I open it, it says I need a 10.13 version. I look up a 10.13 version, but NOTHING shows up on how to download it. No software updates are showing up on the App Store. Any help?