I have not been able to access online help through lightroom. If I select Help->Lightroom Help... from the menu, it takes me to the URL:
And I get an error that the webpage is not available. In previous versions of lightroom, help went to http://helpx.adobe.com/lightroom/topics.html.
Is the URL in my LR CC 2015 incorrect, and if so, is there a way to correct it?
Yes, it would appear that the URL in LR6/LRCC is wrong. Until Adobe fixes it, you should bookmark the old URL that you have.