require/include throwing warning: no such file or directory in

Question

I'm getting a rather frustrating warning which is blocking me from further development.

I've tried using require_once / include_once for a file with a relative path, and it seems to be unable to find it. Then all of a sudden it works, and now it doesn't work.

What could be causing this problem?

Detailed screenshot of my file and paths:

Notice that line 3 works, but line 4 doesn't work.

Warning I'm getting client side:

Thanks.

Update with a screenshot of my finder: