I would like to make a reusable 2013 workflow that can assign an approval task. The Assign A Task action is not available in Sharepoint Designer 2013 and the Workflow Tasks list setting is not available on the properties page for a reusable workflow. Is this by design? I do see Assign a Task for 2013 list workflows and I see the Workflow Tasks setting, as well. I have configured my SharePoint environment for App Permissions and granted them (using the guid permission) to Workflow.
Did you use App Step for this action? Please try to add it just into a Stage and compare the result.
You can try it on different sites and compare the result.
In addition, please provide a screenshot about Action when clicking it.