Setting up Content Synchronization module
$app_root is the top directory of your website. The one where your index.php is located.

Therfore the ../content/sync is a location like this:

- content
    - sync
- public_html
    - index.php
    - (all the Drupal 8 files)


To add to that — this is probably done out of security reasons. Not to store your data from DB with the code. Same reasoning was followed for the Config Management core module.

And lastly - you have to make sure there is such a location on your server. Locate index.php, go one directory up and locate or create the content/sync path and try again.