There are several reasons why your domain may not be considered secure.
1. You have not created an SSL certificate yet. In order for your domain to be considered a secure connection, an SSL certificate must be issued. If you have already connected your domain to your shop, you can generate an SSL certificate on the domain page. Once you’ve done that, it may take some time for the certificate to be issued. You may also see an error message on the domain page. Don't be alarmed by this. Once you have an "https://..." connection in your shop, you will have a valid SSL certificate.
2. You have created the SSL certificate at your domain provider. The SSL certificate must be issued by a different source for it to be trusted. If the SSL certificate is created where the domain is hosted, it will be considered untrusted and therefore not secure. If this is the case, you can follow the steps from step 1 to generate a valid SSL certificate.
If you are having trouble because your own domain is not working, you can find help in the article "Why is my domain not working?".