You don't even say what computer you are using. Tough to help.
But, when you mention aggregate devices, it suggests that you are talking about some kind of Mac.
Try creating the aggregate device as described for Logic. Then, go into Ableton and try and connect to it. If you cannot, you should go to Ableton support for more help.
All we can do is help you create the device – how you use it is beyond our control or knowledge.
