Add custom filed to Wordpress - php

Question

I'm trying to add a custom field to my Wordpress page but it's not working. When I set its value nothing happens on the HTML.
I read the Wordpress documentation and I tried to follow the steps there but something went wrong.
Everything else it's working, like the_title(), the_post_thumbnail()...it's just this custom field that I it's not Working :(
https://wordpress.org/support/article/custom-fields/
https://codex.wordpress.org/Function_Reference/register_post_type
Custom Field:
Functions.php
$supports = array (
'title',
'editor',
'thumbnail',
'custom-fields'
);
HTML
<?php
$args = array ('post_type' => 'produtos');
$loop = new WP_Query( $args );
if ($loop->have_posts() ) {
while ($loop->have_posts() ) {
$loop->the_post();
?>
<div class="col-xl-2 col-lg-3 col-md-3 col-sm-4 col-md-3 col-12">
<div class="produtos-head">
<div class="img-fluid produtos-img">
<?php the_post_thumbnail(); ?>
</div>
</div>
<div class="produtos-titulo d-flex align-items-center justify-content-center">
<?php the_title(); ?>
</div>
<div class="preco-original"> Price:R$
<?php $original_price = get_post_meta($post->ID, 'original_price', true);
if($original_price){ ?>
<p>
<? echo $original_price; ?>
</p>
<?php
}else{
}
?>
</div>
<div class="preco-promocional"> <span> Sale: </span>
<span class="preco-promocional-number"> $ </span>
</div>
<button class="btn-vendedor"> Contact </button>
</div>
<?php
}
}
?>
</div>
</div>

Answer 1

use get_the_ID() instead of $post->ID and need to change <? echo $original_price; ?> to <?php echo $original_price; ?>