Hi,
It is almost impossible to tell only via screenshot, but I am guessing that the n.post in this case is referring to the jQuery.post(..) method. That means, that jQuery is either not loaded properly, or something is unsetting the built in jQuery XHR post() method there.
If you use any plugins/scripts that temper with jQuery version, or replaces any of the functionality, or changes the jQuery release, then it may cause that. Once again, this is almost impossible to tell only by this short error message.
Alternatively, you can change the script compatibility to ‘Minified Scoped’ under the Compatibility Options panel: https://i.imgur.com/hUIDDMZ.png
That will very likely resolve it, but I still recommend checking why the $.post(..) jQuery method is not set.
I hope this helps!