Why it does not work?

<?php
      global $product;
      $price = $product->get_price();
      
        $deadline = 12;
        $rate = 25;

        $rate_sum = ($price / 100) * $rate;
        $total_sum = $price + $rate_sum;
        $base_per_month = $total_sum / $deadline;
 ?>

it also does not work

$product = wc_get_product( $post_id );
$product->get_price();