Fatal Error Call to undefined function WC()

Phew, that’s a strange one…
The WC() function is called either when Woocommerce fires the ‘woocommerce_init’ hook or when the wc settings are saved. This is positively after all woocommerce php (including the WC() function code) has been included.
The only cause I can think of is another plugin or theme doing a ‘woocommerce_init’ before the woocommerce plugin has been properly loaded.
Are you able to scan the php source in your wp-content/ for the string do_action( 'woocommerce_init (or even better the regex do_action\(\s?['"]woocommerce_init). There should be exactly one match in plugins/woocommerce/woocommerce.php line 399.

Hi Angelo,
at least wooCommerce 2.1 is required. See plugin description and this thread as well: https://wordpress.org/support/topic/not-able-to-intall-this-plugin?replies=2