Skilling is open source. So is Drupal, the software Skilling extends.
Yay! It's free!
Right! Free is good.
You can also modify it. Add stuff for your own organization, or just for yourself. A photo of your dog on every page? Go for it. A joke of the day? No problem.
You can't sell Skilling, though. It's free to everyone, not just you.
What about courses I make using Skilling? Could I sell access to them?
Sure, you could do that, because you're not selling the Skilling software itself.
I've never charged anyone for using my courses. In fact, I make all material open, with a Creative Commons license.
You do what's appropriate for you. For example, you could make the material free, but charge for personal feedback. That's one of many options.