Hi Yunus,
It would be best if you had a dataview, as the parent of this reference selector.
The error tells that you need a dataview
Let me know, if you have any more issues,
Hope it helps!
Just to add more details here, if you stumbled into this error while doing the Rapid Mendix course, you may have tried to simply drag & drop the Combo box & accidentally placed it outside of the Data view. Check your Page Explorer and do something like this: