updating OS on a MacBook Pro 2011

I have a MacBook Pro (13-inch, Late 2011) which has Mac OS Sierra 10.12.6 on it. I am trying to download office that is available from my son for his school work however it says it requires version 10.13.0 or later. On the App Store I can only find Mac OS Catalina which it says is not compatible with my MacBook as its too old.

I can't find a way to get 10.13.0 ... or work out if that would be compatible with my 2011 model. Any ideas?