Did you upgrade from Win7/8.1? If so, how long ago did you update?
You can't install 10586 through Windows Update if it's been less than 31 days since you've upgraded.
If you want to upgrade manually, you can do so by using the Media Creation Tool, which will download 10586 and you can choose to do an in-place upgrade from 10240. It will keep all of your files / installed programs and just upgrade Windows to the latest.
I had to do this the other night with my freshly installed Win10 install (not sure why since it was a clean install), and all I had was the 10240 ISO. So I downloaded the 10586 ISO and did the in-place upgrade...and I was back up and running.