Broswer caching error within Hummingbird

Hello there, I am currently trialling your premium product and have tried to use Hummingbird for browser caching. It's currently saying that it wasn't able to successfully cache with the following error

Browser Caching is not working properly:
- Your server may not have the "expires" module enabled (mod_expires for Apache, ngx_http_headers_module for NGINX)
- Another plugin may be interfering with the configuration
If re-checking and restarting does not resolve, please check with your host or open a support ticket with us.

The website I am currently building is here

Anything you can do to help would be greatly appreciated.
Many thanks