IDE 1.8.5 can't see Adafruit_GFX_H

I am using an Arduino Due, IDE 1.8.5, Adafruit TFT 3.5inch, Windows 10 on a Lenovo laptop. The error message is “#include <Adafruit_GFX.h>” I have repeatedly added this file to the sketch. Each time I have done so the Editor asks if I want to replace the prior version. In researching for a fix I learned that a possible solution might be to write a “prototype” in the header. Not being a programmer I don’t know how to write a “prototype” or exactly where to place it. I would appreciate any help with a solution.

georgec5:
I have repeatedly added this file to the sketch.

How did you do that? You’re not supposed to add the file to the sketch. You’re supposed to install the library that contains the file. You can find instructions for installing libraries here:

In another case where it did make sense to add a .h file to a sketch, you would need to use the double quotes syntax with the #include directive, not the angle brackets syntax. The #include “foo.h” syntax causes the local folder to be searched for the file, then the library folders if the file was not found. The #include <foo.h> syntax causes only the library folders to be searched.

georgec5:
In researching for a fix I learned that a possible solution might be to write a “prototype” in the header.

Nope, that has absolutely nothing to do with this error.