I’wp_ob_end_flush_all’ not found or invalid function name

IN THIS PAGE ???? ???

the error is “Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘wp_ob_end_flush_all’ not found or invalid function name in /home/mohamed/public_html/wp-includes/class-wp-hook.php on line 309“

the code line is

	// Avoid the array_slice() if possible.
				if ( 0 == $the_['accepted_args'] ) {
					$value = call_user_func( $the_['function'] );
				} elseif ( $the_['accepted_args'] >= $num_args ) {
					$value = call_user_func_array( $the_['function'], $args );
				} else {
					$value = call_user_func_array( $the_['function'], array_slice( $args, 0, (int) $the_['accepted_args'] ) );
				}