Enabling extensions - directory not found
You need an absolute path from the server's root (/), and I doubt that there's a directory /public_html in your setup.  More common situations are:


/var/www/public_html
/srv/www/yoursite/public_html
/home/someuser/public_html


or something similar.

You can tell by looking in your civicrm.settings.php file that's in wp-content/plugins/civicrm and scrolling down to the file paths.  One of them will be to the templates_c folder, which should normally be in wp-content/plugins/files/civicrm.  That path will be all the way to the root of your server, so you can just copy that and replace templates_c with custom_ext.

Of course, you could also look at the Temporary Files and Custom Files fields in your Directories admin page.  Under normal circumstances, those should also be pointing to sibling folders of your custom_ext folder.

While you're at it, make sure you set the Extension Resource URL under Administer - System Settings - Resource URLs.  You can probably use the Image Upload URL as an example, replacing persist/contribute with custom_ext.

Now, why CiviCRM doesn't just create the extensions folder and record its location when installing is totally beyond me.