Q: How to cancel subscription?

Since you do not indicate what subscription you are trying to cancel, it is hard to say, but the support document that you linked here from is what you use. However, at this time there seem to be some issues with iTunes and the App Stores. According to Apple, http://www.apple.com/support/systemstatus/ there seems to be a problem and they are investigating. Check back to that site to see when it gets corrected.

Managing auto-renewing subscriptions -

http://support.apple.com/kb/HT4098

Late 2012 Mac mini, OS X El Capitan 10.11.6;  Watch, 38 mm silver AL, Watch OS 2.2.1; iPad Air 2 & iPhone 6+, iOS 9.3.3;  Apple Airport Express