This error occurs when WP Rocket cannot access your website to generate Used CSS. The problem stems from connection issues between WP Rocket’s servers and your website. Your site might be behind security measures that block external connections, or it could be in maintenance mode or protected by authentication.
The `WP_ENVIRONMENT_TYPE` constant set to “local” environment can also cause this issue. When WP Rocket cannot access your site, it cannot analyze your CSS usage, which impacts the optimization process.