This error message happens when performing an edit via the API (action=edit). It means that the token parameter must appear in the *body* of a POST request.
Surprisingly (to this dev), you can send a POST request with all of the data in the query string of the URL. FWIW, the API will not accept the 'token' param in the query string of a POST request.
The current error message ("The 'token' parameter must be POSTed") suggest to me that the problem was that the request was not a POST requests (which it obviously was). I think that this message ought to be changed to make it clear that the problem was that the 'token' param did not appear in the *body* of the request.
"The 'token' parameter must be in the POST body."
Or perhaps more verbosely:
"The 'token' parameter was found in the query string, but must be in the POST body."