Google Merchant Centre

I am trying to list all our photoshoot vouchers on Google Merchant Centre - this means linking each voucher web page to the product in Google merchant centre.

However google keeps saying it can't locate "missing inventory data"  - I have gone into each product/voucher and entered stock of 999 - a product id for each voucher that is the same as google product - and a default shipping

Even though I have added these identifiers to each product/voucher page they do not appear to show up on the html when I examine it.

I have added to header code <meta name="availability" content="in_stock"> this does show up, but for every page not just the voucher pages.

How do I get say the id=S27001 to appear in the page without displaying.

I hope this makes sense

Thanks Keith

I have sorted it using JSON

<script type="application/ld+json">

{

  "@context": "https://schema.org/",

  "@type": "Product",

  “title”: "Mother & Child Photoshoot, capture precious moments together forever",

  "description": "One hour portrait session | Cinematic viewing of your images | Beautiful Art-Mount Gift Print",

  "url": "https://www.studio27.photography/gifts/newborn/mother--child-photoshoot/",

  "price": “59.00 GBP”,

  “id”: “S27006”,

  “store_code”: “S27”,

  “availability": “in-stock”

}

</script>