Add Custom Variable to Follow up emails.php

function fue_register_variable_replacements( $var, $email_data, $fue_email, $queue_item ) { 
    $variables = array(
    	'tracking_number'	=> ''
	);

    if ( isset( $email_data['test'] ) && $email_data['test'] ) {

		$tracking = get_post_meta( $order_id, '_wc_acof_2', true );

        $variables['tracking_number'] = "$tracking";

        if ( !empty( $email_data['order_id'] ) ) {
            $order = wc_get_order( $email_data['order_id'] );
            $tracking_number = get_user_meta( $order->customer_user, '_affiliate_key', true );

            if ( !empty( $tracking_number ) ) {
                $variables['tracking_number'] = $tracking_number;
            }
        }
    } else {
        if ( !empty( $queue_item->order_id ) ) {
            $order = wc_get_order( $queue_item->order_id );
            $tracking_number = get_user_meta( $order->customer_user, '_affiliate_key', true );

            if ( !empty( $tracking_number ) ) {
                $variables['tracking_number'] = $tracking_number;
            }
        }
    }

    $var->register( $variables );
}