// Validation - if user hasn't selected a product addon, then alert them to this
// Place this in functions.php
function product_addons_validation( $passed, $product_id ) {
if ( empty( $_REQUEST['addon-' . $product_id . '-field-name'] )) {
    wc_add_notice( __( 'Product-Addon Error Message.', 'woocommerce' ), 'error' );
    $passed = false;
}
return $passed;
}
add_action( 'woocommerce_add_to_cart_validation', 'product_addons_validation', 10, 5 );