It allowed me to save but at runtime, it is throwing exception:
com.pega.pegarules.pub.clipboard.InvalidParameterException: must have mode Page List. Details: Invalid value for aPageList passed to com.pega.pegarules.data.internal.clipboard.Sorter.sortPageList(ClipboardProperty, String, boolean, String)
Obj sort typically needs to know what kind of objects it is sorting because you need to tell it which properties you are sorting on. If you have multiple types of objects in the list, then identify a common ancestor for your .pxresults (assuming you are sorting code-Pega-list). If you are working with an embedded page list in a parent class already defined, then an additional pages and classes reference would not be required. Remember that if you have multiple object types in your list, you should only be able to sort on properties that they have in common and get from a common ancestor which is either identified in your pages and classes or is identified in the parent (container) class.
If you don't provide sort properties or if your embedded classes don't inherit from a common ancestor with those properties, you can expect errors.