The path needs to point exactly to ffmpeg.exe, not just the folder. Ex (C:\Users\YourUsername\WhereYouHaveItSaved\ffmpegVerisonName\bin\ffmpeg.exe) In windows, navigate to it in file explorer, then copy the file path above by clicking it. Then, paste it into the replay mod under "Command Line Settings" and add "ffmpeg.exe" to the end.
Hope this helps!