Hello,
I have Kdevelop (Version 4.2.2) and when I try to build my project I receive this error message twice in a row:

"Internal error: cannot build a file item"

I really don't understand what's wrong, because the project is properly configurated. Any help appreciated, thanks!

If you recreate your project, does this occur?

I solved this problem by taking the "file" (or files) out of the Project Selection box, the one which is on the bottom left corner. You should just have in it the projects that can be built.

Hope it is useful for you.