wprock

You can do this

update_user_meta( $customer_id, 'YOURCUSTOMFIELDKEY',YOURCUSTOMFIELDDATA);

Try this once

<?php

if ( ! defined( 'ABSPATH' ) ) exit; // Exit if accessed directly
   
global $post, $product;

if ( ! $product->is_in_stock() ) return;

$sale_price = get_post_meta( $product->id, '_price', true);
$regular_price = get_post_meta( $product->id, '_regular_price', true);

if ( empty(  $regular_price ) ) { //then this is a variable product
    //LINE BELOW CAUSING FATAL ERROR ON SEARCH PAGES
    $available_variations = $product->get_available_variations();
    $variation_id=$available_variations[0]['variation_id'];
    $variation= new WC_Product_Variation( $variation_id );
    $regular_price = $variation->regular_price;
    $sale_price = $variation->sale_price;
}

$sale = ceil(( ($regular_price - $sale_price) / $regular_price ) * 100);

if ( !empty( $regular_price ) && !empty( $sale_price ) && $regular_price > $sale_price ) : 
?>
    <?php echo apply_filters( 'woocommerce_sale_flash', '<span class="onsale">-' . $sale . '%</span>', $post, $product ); ?>
<?php endif;

You should hire a developer for this issue.This is my opinion.

Looks like there have problems in coding or product variations settings

You can check this tutorial

I tried this snippet

add_action( 'admin_footer', function(){
	$screen       = get_current_screen();
	$screen_id    = $screen ? $screen->id : '';
    if ( ! in_array( $screen_id, array( 'product', 'edit-product' ) ) )
    	return;
?>
    <script type="text/javascript">
        jQuery( function( $ ) {
            $( function() {
                // Attribute ordering.
                $( '.product_attributes' ).sortable({
                    items: '.woocommerce_attribute',
                    disabled: true
                });
                
                $( '.woocommerce_attribute h3 .sort' ).css('cursor', 'pointer');
             });
        });
    </script>
<?php
}, 999);

I added the code into functions.php file

You can try this approach

add_action( 'woocommerce_product_additional_information', 'paulc_custom_field_data', 40 );
function paulc_custom_field_data( $product ) {
	//* WRITE YOUR PHP CODE HERE
}