Is that the actual code you ran to produce that particular error? The error mentions the 'dl.dropboxusercontent.com' hostname, which is a real Dropbox hostname, but isn't what you should be using for this API call, and isn't used in your supplied code anyway.
In any case, for the code you shared, it looks like there are two issues:
You're supplying your API call parameters as a JSON string in a 'data' header. The /2/file_requests/create endpoint is an "RPC"-style endpoint though, so it expects the API call parameters as a JSON string in the request body, not a header.
You're supplying a destination path of "/", but that should be a path to a particular folder.