While indexing your website as Googlebot (choosing the User-Agent - Googlebot from the SEO tool), the result status code is 403 (permission denied).
This might be due to the fact that you are using a WAF (such as CloudFlare) that can identify that this was a fake Googlebot and not the real Google service.
In you want further details on the way Google verify their bots you can check the article https://support.google.com/webmasters/answer/80553?hl=en