I think this is my fault. I removed or deprecated the Guzzle6 package at Lean, Flexible, Community-Made Extensions | MODX Extras because it had a security issue and is out of date. I switched other packages to Mark Hamstra’s Guzzle7 extra, but forgot that Guzzle6 was used by RefreshCache.
I’ll try to find time to work on an updated version of RC.
In the meantime, you might try just installing Mark’s Guzzle7 package. It should autoload the classes once installed. Comment out or remove line 11 of the core\components\refreshcache\processors\refresh.class.php file. This line:
require_once MODX_CORE_PATH . 'components/guzzle6/vendor/autoload.php';