I'm sorry but I don't know what getdiskspacefree is. I don't believe it has anything to do with the Camtasia Program. If you could provide more details on how you acquired Camtasia that would be very helpful.
Did you get Camtasia 8.6 through TechSmith?
Have you purchased a activation key?
I would download the Camtasia 8.6 trial version directly from TechSmith and install it. If you have a activation key you can use it or run the trial version. https://www.techsmith.com/camtasia.html