Ok, my understanding was that you’re building a drag/drop functionality within the same repeating group, or within nested repeating groups with data linking (where cells can be linked to each other.
But you have an external repeating group with a list of tasks, which you want to drag over to a different repeating group. This is not possible, not with the Draggable elements plugin.
There might be other plugins that help with that, for example Drag it - RG->RG paid plugin, but I have not used that one.