Hi Daniella,
I see you’re using WordPress version 4.9.5, We generally only support as far back as three major releases of WordPress. Please update your Wordpress to the latest version (5.3 as of this writing).
Release Notes - Pro 3.1, X 7.1, Cornerstone 4.1
In this case specifically, WordPress added many javascript dependencies like React, ReactDOM and Babel part of WordPress core. It is best practice to always use libraries bundled with WordPress when possible, which is why they were removed from Pro (and X/Cornerstone). This is why Pro (Builder/Cornerstone) doesn’t load, because many of the required scripts are unavailable.
Updating WordPress
Remember to clear all your caching features (plugin, server-side, CDN, and browser’s cache) after updating so that the code from the latest release is always in use. This will help you to avoid any potential errors.
Hope it helps,
Cheers!