If you’re using Visual studio, right click on the project and then on Properties, Under Configuration Properties click on CC++ and then add the directory to your header files under the Additional Include Directories section.
There is more information here on how to deal with this problem: Where does Visual Studio look for C++ header files?
For me, I followed xflowXen’s answer and then at “Include Directories” typed in the specific pathname where my header file was located followed by a semicolon, something like:
C:Usersnamesourcereposp2-Ap2-A;
then applied the changes and the issue went away.