The attribute should have been added in step 7.9.1:
Go ahead and add the attribute TotalNumberOfRegistrations to the TrainingEvent entity. As explained in the previous lecture, it should be a stored attribute of the type Integer.
You might have skipped the step, or added this to the incorrect entity, created the attribute as a calculated attribute or you selected the incorrect entity in the change activity action. These are some of the causes for not being able to select the attribute in the change activity.
Please check your domain model to verify step 7.9.1 was performed as described and if so then look at some of the other possible causes.