This is quite common and happens when there are no outbound links found on your post, which is needed to get a green bullet. Please note that your site needs to contain outbound links. Having no outbound links could negatively affect your ranking. These links provide direct access to another domain within your site.
Unfollow and Follow Links
Every link on your site is followed by Google by default since it helps to gauge SEO rankings. If there are any links that you don’t want Google to follow, you can set it as nofollow link, which essentially is an indication to the search engine to not follow links on the page. This is usually done so that authority is not passed onto a site. Follow links are the opposite and pass on authority to a website.