Thanks for asking.
You are seeing a PHP warning message. You can get rid of this message by turning off the debug mode. You can do this by opening
wp-config.php and adding/changing:
When you revisit the page, you should no longer be seeing the message.
Also, if you are using Jetpack plugin on the website, please try deactivating the same and see how it goes.
Does that mean you don't see Cornerstone edit button on post? If yes, you can activate Cornerstone on posts from X > Settings > Permissions and click on Posts to activate Cornerstone.
To learn about Permission manager, please take a look at following article.