Current Version is: OS X el Capitan 10.11.6 What's Next?
The highest version of macOS you might be able to run is High Sierra - why not download that installer or try sierra? 


https://support.apple.com/kb/HT208202
http://updates-http.cdn-apple.com/2019/cert/061-39476-20191023-48f365f4-0015-4c41-9f44-39d3d2aca067/InstallOS.dmg


As commented - we have several questions on how to download specific versions if that’s the thing that’s blocking you.


How can I download an older version of OS X/macOS?