add_action( 'woocommerce_checkout_process', 'wpslash_tipping_mandatory', 88888 );
function wpslash_tipping_mandatory() {
global $woocommerce;
$tip = WC()->session->get( 'wpslash_tip_selected' );
if(!$tip)
{
wc_add_notice(
__( 'Sorry, but you have to select a tip ', 'wpslash-tipping' ), 'error'
);
}
}