The place to add this will be in product.liquid under ‘offers’.
The liquid file outputs information about the product being viewed, no matter which product is being viewed. Google looks through this data to understand the variables about each product and if a variable is missing or doesn’t translate exactly between Shopify and Google, you get the error.
The error message Missing Field “ShippingDetails” (In “Offers”) is literally telling us that in the product.liquid file, under ‘offers’ there are no shipping details defined.
Please send me a private message if you’d like more help.