The error means you are using filename as a variable or function but have not created it or defined it yet. Given the choice of name, I suspect you intended it to be a variable. Look at your code (the error message will tell you exactly what line) and see if you ever assign a value to filename.
If that's not it, we can't do much more than guess unless you are willing to share your code. Also, it's best to share the full error message (copy/paste all the red text) here.
