Hi,
I think you simply did not import vuforia properly, unfortunatly, vuforia packaging is careless and they don't pauy attention for backward compatibility, so
1:create a fresh project,
2:import vuforia,
3: import PlayMaker
4: import the vuforia package from ecosystem
If that works, then you'll need to clean up your existing project from the old vuforia stuff... pain.
I updated the github repository using the latest 6.2 and it was fine ( not 2017.2 as it's now shipped with Unity).
Bye,
Jean