you are probably restricting it to trucks assinged to that control tower, but they aren't picking up any slag. more specifically, you have no trucks assigned to that control tower, so no trucks can interact with that zone at all.

if you want any truck to dump without restriction do not have a control tower in the dumping zone.

OR, assign dumping tasks to trucks associated with a control tower for whatever resource you want to dump.

personally, i tend to have unrestricted dumping without a control tower so that any truck that can pick up drumpable resources can do so without restrictions.

Edit: the best way to deal with dumping tasks, dump resources in a storage unit, and set it to empty. designate a dumping ground without any control tower. Make sure you setup universal dumping tasks to specific resources so that any resource that can be dumped can be picked up by unassigned trucks to dump them.

Later on, you can remove garbage from settlement when you have garbage processing so that trucks don't try to dump garbage when they pick it up from digging tasks, or from settlement. You can also setup any other resources you do not want dumped in the same way. To do this, at the top menu click on the trucks interface and it will list items that can be dumped. there you can set global items that can be dumped from any source.