This is from the Bubble Doc’s. I think whatever that “This RepeatingGroup’s List of Type appartements:first item’s levertijd’s dagen” equates to needs to be a String or Text that is the name of one of the fields for that Data Type. Is that true.
Dynamic field name
If you want the search to be sorted according to a field chosen by the user, define an expression to define which field the search should use. For instance, the expression you design here should evaluate to ‘Created Date’ or some other fields you have defined in your type (it should match the displayed name of the field).