That’s not how the Draggable elements plugin works. The plugin offers 1 functionality only, when a DropArea has a group dropped on it and a workflow is associated with this drop area, then the workflow will run.
The tasks you want to drag reside within an external repeating group. A repeating group draggable cell’s value cannot be accessed outside of the Repeating group it resides in (not by dragging it). We won’t know which task we’re dragging so we cannot assign that task.