It means you've written your custom module incorrectly. You either have a module name which doesn't match the folder you've put it in, or you have a name with capital letters or spaces, or something like that.
Also, a typo when compiling would do it. It never says "file not found", it would give this error message you're getting.
Also, a typo when compiling would do it. It never says "file not found", it would give this error message you're getting.