subprocess "TypeError: a bytes-like object is required, not 'str'"

subprocess returns bytes objects for stdout or stderr streams by default. That means you also need to use bytes objects in operations against these objects. "Duration" in x uses str object. Use a bytes literal (note the b prefix):

return [x for x in result.stdout.readlines() if b"Duration" in x]

or decode your data first, if you know the encoding used (usually, the locale default, but you could set LC_ALL or more specific locale environment variables for the subprocess):

return [x for x in result.stdout.read().decode(encoding).splitlines(True)
        if "Duration" in x]

The alternative is to tell subprocess.Popen() to decode the data to Unicode strings by setting the encoding argument to a suitable codec:

result = subprocess.Popen(
    ["ffprobe", filename],
    stdout=subprocess.PIPE, stderr = subprocess.STDOUT,
    encoding='utf8'
)

If you set text=True (Python 3.7 and up, in previous versions this version is called universal_newlines) you also enable decoding, using your system default codec, the same one that is used for open() calls. In this mode, the pipes are line buffered by default.

Like the errror says, "Duration" is a string. Whereas, the X is a byte like object as results.stdout.readlines() reads the lines in the output as bytecode and not string.

Hence store "Duration" in a variable, say str_var and encode it into a byte array object using str_var.encode('utf-8').

Refer to [this][1].

[1] : Best way to convert string to bytes in Python 3?