I could not create an Interview because I could not create and Interview Round because I could not add a Skill to the Expected Skillset child table because the child table tries to retrieve the Description for the selected skill but that field does not exist in my docType. Not sure how that happened - maybe apps I have installed have the same docType Skill but one table does not include the Description field.
The solution was to create a Custom field on the Skill docType called ‘Description’.