Yes it should work – it has been tested up to 5.8.2 but 4.6.19 is 18 months old, why not update WordPress?

To force an update of Scalability Pro, you can download the latest zip file from https://www.superspeedyplugins.com/my-account/downloads/ then visit Plugins > Add New > Upload > choose the zip file then confirm to replace the existing plugin.

To be fair, this might not work on such an old version of WordPress – any particular reason you’re on an 18-month old version of WordPress?

=== Scalability Pro ===

Contributors: dhilditch

Donate link: https://www.superspeedyplugins.com/

Tags: speed, performance

Requires at least: 4.6

Tested up to: 5.8.2

Stable tag: 4.76

Requires PHP: 7.0

License: GPLv2 or later

License URI: https://www.gnu.org/licenses/gpl-2.0.html