If you’re seeing this message, there’s a simple PHP snippet you can add to your functions.php file courtesy of Kevin Leary:
https://www.kevinleary.net/wordpress-ob_end_flush-error-fix/
/* Prevent ob_end_flush() notice when in debug mode */
if (WP_DEBUG) {
remove_action( 'shutdown', 'wp_ob_end_flush_all', 1 );
add_action( 'shutdown', function() {
while ( @ob_end_flush() );
} );
}