Change $ to jQuery of $( document ).ready(function() at /themes/neuron/assets/js/ajaxchimp-config.js

/* === Mail Chimp subscription form settings === */
jQuery ( document ).ready(function($) {

    $('.mailchimp').ajaxChimp({
        callback: mailchimpCallback,
        //replace bellow url with your own mailchimp form post url inside the url: "---".
        url: "http://trendytheme.us9.list-manage.com/subscribe/post?u=85ba3666ffb519396fbe64dc5&id=c335e5ec53" 
    }); 
    function mailchimpCallback(resp) {
         if (resp.result === 'success') {
            $('.subscription-success').html('<i class="fa fa-check"></i>' + resp.msg).fadeIn(1000);
            $('.subscription-error').fadeOut(500);
            
        } else if(resp.result === 'error') {
            $('.subscription-error').html('<i class="fa fa-times"></i>' + resp.msg).fadeIn(1000);
        }  
    }

});