Greetings, answering your questions:
it has certificate errors for secure (https)
A Secure certificate to support TLS in a custom domain related to your app is avaliable as always as you have enabled the white label addon, this option is not avaibl by default for any new custom domain.
what does the “Click here to request an HTTPS” certificate supposed to do? as this isn’t very well documented.
You should use this option if you have already set a custom domain pointing to your app and you wish to deploy a TLS certificate to it in order to support https. Once you use it, a member of the DevOps will take your request and will deploy within 48 hours a secure certificate for your custom domain as always as you have already enabled the white label addon.
Let me know if you have any additional question.