Once you've added a custom domain to the sGTM container and opened this URL, you might see an error your connection is not secure. This error indicates issues with the SSL certificate. 

There might be three reasons for this error to appear.

1. Custom domain is not verified on stape. To check it, log in to your stape.io account. Open sGTM container settings and check if the tagging server URL status is Ready. If the status is Verifying, follow the steps from this article. 

2. It usually takes 30 minutes to issue an SSL certificate for your domain. If there were less than 30 minutes, please wait and check the status after. 

3. If non of the two options above apply to your situation, contact support.