Fatal error: Uncaught Error: Call to undefined function is_product()

Solution:1

First check if woocommerce activated then call function:

add_action('wp_head', 'wc_variable_simple_conditions');
function wc_variable_simple_conditions(){
    if (!class_exists('WooCommerce')) return; // add this line
    if (is_product()) {
        global $post;
        $post_id = $post->ID;
        $product = wc_get_product($post_id);
        $type    = $product->get_type();
        if ($type == 'variable') {
            add_filter('woocommerce_available_variation', 'load_variation_settings_fields');
        } elseif ($type == 'simple') {
            add_action('woocommerce_single_product_summary', 'custom_data_above_add_to_cart_button', 41);
        }
    }
}

Solution:2

Make sure that you include source of this function includes/wc-conditional-functions.php

http://woocommerce.wp-a2z.org/oik_api/is_product/