When publishing your products to Google Shopping, it is a requirement that the product pages are accessible and meet certain quality requirements that Google imposes, which are:

• Show all key elements of your product
• If linking to a variation, the correct variation must be shown
• The product price must be clearly shown and must match the advertised price

More detailed can be found in this Google help article.

If your site is in pre-release (under construction), protected by password authentication or not publicly accessible, it won't be suitable to use with Google Shopping ads.