"unsupported_grant_type" response when attempting to obtain authorization token

[todd7]

I've followed the directions here:

https://medium.com/@AbraDevelopment/canvas-lms-oauth2-f83c98e6e805 

and here:

OAuth2 - Canvas LMS REST API Documentation 

when posting the code I receive during the flow to /login/oauth2/token I get the following response:

{

   "error": "unsupported_grant_type",

   "error_description": "The grant_type you requested is not currently supported"

}

I've tried it on localhost and on an EC2 instance and I get the same error every time. Is this a common error? What could be the cause?