Typically this error results from either of the following:
Your meta tag is not properly placed on your website and therefore Google cannot find it when it crawls your pages
Ensure that your meta tag has been installed properly on your site. This is done differently based on your ecommerce platform/provider.
Your website is temporarily down such as for maintenance or it hasn't been launched yet making it impossible for Google to crawl your pages
Launch your website or exit maintenance before attempting to claim and verify again.
Your website URL in your Google Merchant Center account is incorrect or not formatted properly.
Log into Merchant Center, go to Business Information and then to Website. If your URL was incorrect, update it before attempting to claim and verify again.