More than likely, you are not running SketchUp 2017 as an administrator. In order to make a directory anywhere under the Program Files folder, you must have admin rights in Windows. In general, storing user documents of any type under Program Files is discouraged by Microsoft. We finally corrected where SketchUp reads and writes content files (like materials) in SketchUp 2017.
If you are going use a custom Materials folder other than the SketchUp 2017 default, I would suggest placing it under your Documents folder where you have read and write rights, even if not running as an administrator.
Hope that helps,
Tyler