I have a Fortran program I am debuging. I have a list of varables and one of the expected variables is almost double its expected value. So I compiled the program, with debug f
The error means that the source file w_powf.c is not available on your system. It's not related to your program apparently. You can safely ignore that error.
Note that the lines you mention have the operator ** which I suppose is implemented in that file, so it's just telling you that there is no access to the source code of the implementation of that operator.