Such warning you can get just FIRST time after installation. Just save settings and change currency on the front and this notice will gone. If not - that is mean that you have wrong code options in the plugin settings. Just look this screen in doc to understand how currencies codes should be registered.
After making options right - just change currency on the front to be sure that all done well ...
This notice is triggered in index.php of the plugin in the function woocommerce_currency_symbol