Fatal error: Uncaught Error: Call to undefined function wp_get_sites()

The PHP runtime stopped because it could not find the function wp_get_sites(). WordPress loads this function only in multisite environments and only in versions prior to 4.6. When the code calls the function on a newer version or on a single‑site install, the function is never defined and PHP throws a fatal error.

The error appears as a blank page or a 500 response. It stops the request before any HTML is sent, so the dashboard and front‑end become inaccessible.