When setting up a trial of the marketing app, the setup experience will show orgs that are available in your subscription for trying out the marketing app. Included in this list are CDS instances, which will not work since they do not have many of the core system entities that the marketing app relies on (such as leads, marketing lists, etc.). If a person tries to deploy a marketing trial on a CDS instance, it will fail after several hours with the message "System deployment failed." Therefore, CDS instances should not be displayed as an option for setting up marketing trials.