High Sierra will not upgrade from 10.13.6

I have a MacBook Pro (Retina, 13-inch, Early 2015) and system is running on 10.13.6 (17G14042).

The system is not showing any updates in the App store. It says "no updates available" in the tab "updates". I read some stuff, but could not find a solution. I have enough space (> 100GB) available for an upgrade, which would also be fine for me. Also no upgrade is showing up in the app store in the "highlights" tab.

I have some software that runs on 11 or higher only.