Cannot download update past Version: 10.11.6

Hello, I am a college student in need of some assistance. One of my classes uses Microsoft Powerpoint, Excel, Word, etc. and I am unable to download these apps. The message "(null) can’t be installed on “Macintosh HD” because OS X version 10.12 or later is required" pops up when I try and download these apps. When I check for updates, the computer shows it is up to date. Any help on how to download version 10.12 would be greatly appreciated. Thank you!