Check the "grant_type" parameter
<p>I am using <code>OAuth 2.0</code> for authorization according to this documentation :(<a href="https://developers.vendhq.com/documentation/oauth.html#oauth" rel="nofollow noreferrer">https://developers.vendhq.com/documentation/oauth.html#oauth</a>) and having this error:</p>

<blockquote>
  <p>"error": "invalid_request", "error_description": "The request is missing a required parameter, includes an invalid parameter value, includes a parameter more than once, or is otherwise malformed. Check the \"grant_type\" parameter."</p>
</blockquote>

<p><strong>Request</strong></p>

<p>Method : <strong>POST</strong></p>

<pre><code>Content-Type: application/x-www-form-urlencoded

URL : https://{domain_prefix}.vendhq.com/api/1.0/token
</code></pre>

<p><strong>Parameters :</strong></p>

<pre><code>code          = {code}

client_id     = {app_id}

client_secret = {app_secret}

grant_type    = authorization_code

redirect_uri  = {redirect_uri}
</code></pre>