https://www.drupal.org/forum/support/post-installation/2015-01-30/how-to-edit-settingsphp
After updating Drupal Core if you encountered error displayed in image above make sure you have cleared the cache.
This is also possible that you can’t access the /admin/config therefore access performance link directly at this url /admin/config/development/performance
In my case I upgraded Drupal Core and got The website encountered an unexpected error upon accessing the /admin/config.
In order to understand the cause of this issue, I added following line of code to the sites/default/settings.php
$config['system.logging']['error_level']='verbose';
And issue was resolved by simply clearing the cache, in your case error could be different.
