Error cache.php on line 176

I have an affiliate link from Hotels combined that does not show up on my site any more. However when I clear the cache, the advert appears once, but the following error then shows at the bottom of the page.

Fatal error: Call to undefined function request_filesystem_credentials() in /home/public_html/wp-content/plugins/jch-optimize/platform/cache.php on line 176

Any ideas why?