In Xcode7 and earlier versions, the commenting option was available in Editor > Structure > Comment Selection. In Xcode 8 this option is disabled.
To enable these options run sudo /usr/libexec/xpccachectl and restart your Mac.
There is thread on Apple Developer Forums regarding this issue and several possible fixes. For me running the above command and restarting the Mac did the fix.
Also in Xcode 8 release note it states:
To use the Editor’s Comment/Uncomment Selection and Add Documentation commands—as well as other installed Xcode Extensions—on OS X version 10.11, launch Xcode and install additional system components, then restart your Mac. (26106213)