Are you referring to this issue in the screenshot below?
It's highly likely that the Portfolio page was deleted. Please set up a Porfolio Page. You can read the instruction here: https://theme.co/apex/forum/t/features-portfolios/98
Regarding your issue with Cornerstone, please do try the solutions below:
If you're using a caching plugin, clear all caches including browser cache then deactivate your caching plugin and other optimization plugins.
If you're using a CDN, please clear the CDN's cache and disable optimization services.
Test for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it's fixed, you'll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
Contact your host to increase your allocated memory or do it yourself by adding this code in your wp-config.php
define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );
If none of that helps, please open a separate thread for this Cornerstone issue as mixing topics in a thread mostly leads to confusion. In the separate thread, give us your site's WordPress Login URL, Username and Password and also FTP Host Name, Username and Password in a Secure Note
Hope that helps.