why cant i update my macbook any newer than monteray 12.7.2

I want to download GarageBand on my 2016 MacBook but I don't have the required iOS update. When I look for updates it says I am already up to date, Monterey 12.7.2. I need at least iOS 13.5 (ish) to download GarageBand. I have loads of disc space left so not sure what I need do??