Can't concat bytes to str

subprocess.check_output() returns a bytestring.

In Python 3, there's no implicit conversion between unicode (str) objects and bytes objects. If you know the encoding of the output, you can .decode() it to get a string, or you can turn the \n you want to add to bytes with "\n".encode('ascii')

subprocess.check_output() returns bytes.

so you need to convert '\n' to bytes as well:

 f.write (plaintext + b'\n')

hope this helps