1)  You must add a Section to hold the checklist to the sn_hr_core_template form as described here: Link

Under "Configure - Form Layout - Section - New", Create a new form section.

Then with that section selected, scroll all the way down in the Available fields list and add "Checklist" to the section.

Screenshot:

2) Now you will have a checklist section on your "HR Template" form, so you can actually change the "HR task type" to "Checklist" and save the template.  Screenshot:

3) Finally, understand that at runtime, when the Case Task is created, the checklist section STILL will not appear within the task until the user clicks the actual checklist icon way over on the right.  Screenshot:

Please mark as helpful if this helped you.  : )