It’s giving an error because FFmpeg requires that an output file be specified. Using it just to get information about a file isn’t its intended use.
Option 1: Ignore the error. FFmpeg prints the file information first, so you can simply get the information you need and ignore the error.
Option 2: Use ffprobe instead. FFprobe is another tool usually packaged with FFmpeg that’s designed for getting information about media files. It can even output the information in a variety of easily parsed formats so you don’t have to mess around parsing FFmpeg’s output.