1. Mobile verification not enabled or enabled for less then 7 days.
2. Private profile or inventory.
3. Invalid trade url.
4. Problem with Steam or our trade bots.
5. Problem with specific item(s).

If you are sure about the first 3 then point 4 should be the case and it should be working again after little waiting.

If point 5 is the case, its likely that items you are trying to withdraw/deposit are no longer available and you should choose another item.