Portals do not have found sets and cannot be sorted by the sort records by field script step nor by the sort records script step. Both sort the layout's found set, not the set of related records shown in the portal. The records displayed in a portal cannot be dynamically sorted in that fashion.
There is a way to do that with a portal sorted on an unstored calculation that evaluates differently depending on the value you set in a global field or you can set up a series of tab or slide control panels and put a copy of the portal in each panel but with a different sort order specified in Portal Setup. The user then "sorts" the portal records by choosing a different panel of the control.