This error occurs when WordPress tries to call the function wp_get_active_and_valid_themes() but the function is not defined in the current execution context. The function lives in the core file wp-includes/load.php. If that file is missing, corrupted, or never loaded, the call fails and PHP throws a fatal error.
Typical triggers include a broken core installation, an incomplete upgrade that leaves mixed version files, or custom scripts that invoke the function without loading WordPress first. A plugin or theme that aborts execution before core files load can also prevent the function from being defined.