{"code":"woocommerce_rest_product_not_created","message":"L\u2019UGS (1-XXXX) que vous tentez d\u2019ins\u00e9rer est d\u00e9j\u00e0 en cours de traitement","data":{"status":400}}

And no UGS in product page of woocommerce.

Because I had the same issue, the SQL requests. Make a backup. :

-- Check
select * from wp_postmeta left join wp_posts
on wp_postmeta.post_id = wp_posts.ID
where wp_posts.ID IS NULL;

-- Delete
delete wp_postmeta from wp_postmeta left join wp_posts
on wp_postmeta.post_id = wp_posts.ID
where wp_posts.ID IS NULL;

Delete orphan variation

-- Check
SELECT * FROM `wp_posts` o
LEFT OUTER JOIN `wp_posts` r
ON o.post_parent = r.ID
WHERE r.id IS null AND o.post_type = 'product_variation'

-- Delete
DELETE o FROM `wp_posts` o
LEFT OUTER JOIN `wp_posts` r
ON o.post_parent = r.ID
WHERE r.id IS null AND o.post_type = 'product_variation'