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

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);        }    }}

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

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

Try replacing your function with this:

function wc_variable_simple_conditions(){    if ( !function_exists( 'is_product' ) ) {        return;    }    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);        }    }}