This error is most commonly caused by a browser or browser setting that blocks "third-party cookies." Like many Blackboard integrations, Yellowdig uses browser cookies to connect your Blackboard account to your Yellowdig account. By default, Safari blocks third-party cookies, and most browsers have a setting that can be used to block them. To access Yellowdig, you need to either choose a browser that does not block third-party cookies by default (e.g. Chrome) or configure your browser to allow them. See this Yellowdig support article for details on how to allow third-party cookies in most popular browsers.