This error appears when the environmental variable, WICED_STUDIO_SDK_PATH, is not defined. Please set WICED_STUDIO_SDK_PATH to /Users/your-username-here/Documents/WICED-Studio-6.4/ (default SDK directory).
To do this, in WICED Studio 6.4, go to Project -> Properties -> C/C++ Build -> Environment.
Hit Add... on the right side of the pane.
Set Name to WICED_STUDIO_SDK_PATH
Set Value to /Users/your-username-here/Documents/WICED-Studio-6.4/
Hit Ok -> Apply -> Ok
This should fix it. Feel free to let me know if there are further issues. Thank you for your patience.