Making a HTTP Request using Workfront API v18.0

I am currently trying to use the Workfront API v18.0 in order to upload an image to a client Workfront Sandbox instance. While testing this new version of the API, I started with trying to make a  simple GET request of an Issue in the Sandbox instance using the following URL and Headers:

URL: https://<instance>.workfront.adobe.com/attask/api/v18.0/issue/<issue_id>

Headers: apiKey: <apiKey>, Cookie: wf-node=<node ID>

When making this call I am receiving the following response:

{

   "error":{

      "message": "You are not currently logged in."

   }

}

Is anyone familiar with what this error means and how I can prevent it from happening?

Thank you!