OK, this is an error message of the server. It has nothing to do with Python. You need to learn about what exactly the API expects in your request.
The text I have shown to you was just an example. I have no idea what your server accepts as requests.
In my example the trailing newline is just a whitespace which does not change the data inside JSON. If you need to have a newline inside a string inside JSON you can use the two literal characters \n which means you will need to double the backslash to tell Python to produce a single regular backslash (instead of treating it as a special character).
Let’s take my example and add \n to the property value of firstName:
data = '{"firstName": "John\\n"}\n'
# ^^--- These two characters create a newline
# character in the resulting string.
# ^^--- The two backslashes create a single literal
# literal backslash \ in the resulting string.
# The characters \n are then interpreted in JSON
# as a newline character.