You need to ensure that the actions Create Child Incident and Add Parent Incident are available at the right statuses. Specifically, the current status of your single incident returned by that query needs to have Create Child Incident available as an optional action and the Incident process needs to have Add Parent Incident available at the Open status.
Thank you for your response Hadyn. The piece I was missing was the Add Parent Incident on the Open status. Once I added that to my process it worked fine.