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- The simple explanation would be that you simply don't have write access to the path, which is one possibility.

- Another weird phenomenon that may happen is that you are running a very old C compiler and it can't find `fopen` - because you _did_ forget to `#include <stdio.h>`. 

  When this happens on modern compilers, you get a compiler error. On very old compilers (the "ANSI/C90 standard") the compiler would instead cook up some dumb initiative of its own and treat the function as if it returns `int`. This could create very strange and subtle bugs.

  You could avoid such bugs by updating to the latest version of your C compiler. In case of gcc, clang or icc compilers you should also follow this advise: [What compiler options are recommended for beginners learning C?](https://software.codidact.com/posts/282565)

Unrelated to your question `fflush(stdin);` is undefined behavior and rarely the correct solution. Instead you should just ensure to consume the line feed character after taking the input. (For example by a `getchar()` call or by adding a space at the beginning of the `scanf` format string - but this is a topic for another thread.)