I could only get the error with cs6
app.findTextPreferences.appliedNumberingList =app.activeDocument.numberingLists.itemByName("[Default]")
app.findTextPreferences.appliedNumberingList = "[Default]";
app.findTextPreferences.appliedNumberingList = "[Default]"; worked fine
so did
app.findTextPreferences.appliedNumberingList =app.activeDocument.numberingLists.itemByName("[Default]").name;
so give that a go.
The error message in full:
Error: Invalid value for set property 'appliedNumberingList'. Expected NumberingList, String or NothingEnum enumerator, but received NumberingList.