I was able to build and load lab15 fine. There are a number of things in the instructions that you need to make sure to follow. One thing that I saw missing from the instructions in lab15 was setting up a build variable for the TivaWare install location. It setup the linked path variable but not the build variable. I believe whomever wrote the workshop was expecting people to complete some of the earlier labs which included that step. If you go to 2-18 in the instructions it shows how to setup the build variable.
Regards,
John