Replace “The Past Memory should make a single comment on each such post naming a secretly random Memento from among those for which the Amnesiac’s Role is an Owner (if any such Mementoes exist): upon doing so, that Amnesiac begins to openly carry that Memento if they aren’t already doing so.”

with:-

If the Amnesiac’s Role is an Owner of at least one Memento, The Past Memory should make a single comment on that post naming a secretly random such Memento: upon doing so, that Amnesiac begins to openly carry that Memento if they aren’t already doing so. Otherwise, the Past Memory should make a single comment on that post indicating that no such Memento exists.