Unable to update OS X 10.11.2 to El Capitan on my iMac

Hey,

I've just bought an iMac (21,5", QC i5 2,9GHz/8, 1TB) from a second hand dealer, running a company and hadn't used the iMac much since buying it in late 2013. Now I'm stuck trying to download El Capitan onto the harddrive. According to what it says under Devices on my Apple ID, it's running macOS X 10.11.2 at the moment. I would like to update it to High Sierra.

Could somebody please help me, what's wrong here?

Edvin Arleskär, Stockholm