When defining the interview plan for a job within Greenhouse, it's possible to indicate that certain interviews shouldn't be scheduled. When this is done, that interview cannot be scheduled. Because of this, if we see that an interview doesn't have scheduling enabled in Greenhouse, we won't allow you to schedule that interview in InterviewPlanner either.
If you do want to schedule that interview, you'll need to enable that interview for scheduling in Greenhouse. You can do that by going to that interview's "Edit Interview Kit" page. You can get there by going to the job's "Interview Plan" page and clicking on the "Edit" button for the unschedulable interview.
Once you're on the "Edit Interview Kit" page for that interview, you'll need to enable the checkbox that's labeled "This interview needs scheduling."
Once you enable that checkbox, click the "Save" button. Next time InterviewPlanner syncs information from Greenhouse (which happens every few minutes), it will pick up these changes, and you'll be able to attach an interview template to this interview and schedule it.