How can i make custom field value required ( compulsory ) in woocommerce product page when adding product

For the required HTML5 attribute and other custom attributes woocommerce_wp_text_input() function has custom_attributes option.

woocommerce_wp_text_input(  array(    'id' => 'special_price',    'label' => __( 'Wholesaler Price *', 'woocommerce' ),    'placeholder' => '',    'desc_tip' => 'true',    'custom_attributes' => array( 'required' => 'required' ),    'description' => __( 'Enter wholesaler price here.', 'woocommerce' )  ));

You can modify the following code as per your need.

// Validate when adding to cart        add_filter( 'woocommerce_add_to_cart_validation', 'woocommerce_add_to_cart_validation_custom', 10, 3 );/  validation    function woocommerce_add_to_cart_validation_custom($passed, $product_id, $qty){        global $woocommerce;$option = ''; // your custom field's name   if( isset($_POST[sanitize_title($option)]) && $_POST[sanitize_title($option)] == '' )        $passed = false;    if (!$passed)        $woocommerce->add_error( sprintf( __('"%s" is a required field.', 'woocommerce'), $option) );        return $passed;    }

For even more options while adding a product in the cart you may find How to add a custom text box value to cart session array in Woocommerce my this answer helpful.