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' 
            );
    }

      

    }