At a guess -- I don't think this is possible for a number of reasons.
1) https://experienceleague.adobe.com/en/docs/workfront/using/reporting/reports/text-mode/reference-col... The planned hours for a specific role ID would be the hours allotted to the role ID as one assignment on your template task. Assignments are a collection on the task; and the tasks are a collection of the project -- if you scroll down to the section on limitations, one of them is that you can't build a collection of a collection -- so you can't build a collection of planned hours from a collection of tasks.
2) I don't see it listed as a limitation but -- you also can't do a sum total of the planned hours in a collection.
The closest you'd be able to get to this is as a template task report, showing planned hours for every task that has that role listed. Granted, if the role is the only assignee on the template task, then this would be the one case where you could group by template name, and get your total in that way.
