The Plus icon is usually disabled because a revision is already open for the CDH standard release. In this module, Pega allows only one open revision at a time, so you cannot create another one until the current revision is completed or removed. Open the existing revision in Revision Management and continue using that same revision for the exercise. If the training data is out of sync, use the exercise reset option in the module so the open revision is cleared. After that, the Plus icon will become available again.