This version of Unity is unsupported.
Removed

ShaderIncludePathAttribute

class in UnityEditor

Obsolete [ShaderIncludePath] attribute is no longer supported. Your shader library must be under the Assets folder or in a package. To include shader headers directly from a package, use #include "Packages/<package name>/<path to your header file>".

Description

This attribute is no longer supported.

Shader libraries in arbitrary locations are no longer supported. Your shader files must be located under the Assets folder or in a package. To include shader headers from packages, use the following syntax in the shader code:
#include "Packages/<package name>/<path to your header file>"