When you create/populate LIST_ITEM with record group you MUST have 2 elements in each row:
Data value and list item:

DECLARE
group_id RecordGroup;
list_id ITEM;
BEGIN
group_id := Create_Group_From_Query ('my_group', 'SELECT DeptName, DeptName FROM Department');
list_id := Find_Item('DeptId');
IF (Populate_Group(group_id) = 0) THEN
Populate_List(list_id, group_id);
END IF;
END;


SELECT DeptName, DeptName FROM Department
or
SELECT DeptName, DeptNo FROM Department