We realized that the reason it isn't embedding is because it is not SSL certified and is only http. Could you help us with getting a CSR with a trusted authority for our tableau server?
I suspect you're right that getting HTTPS is important here, even on an internal server it's a good idea to implement SSL if there's any sensitivity. However, the problem may also involve cross-domain. Authentication is a serious challenge when embedding content, so Tableau is heavily locked down so you can only embed from trusted domains, or that it will throw pop-up messages to do its own authentication.
I can see the domain of your Tableau server is http://spr-aengine-01.kmc.int
Where is your html? If on a web server, what is that server's domain?
What is your authentication method? (AD, Local, etc?)
Does this happen if the Tableau user is already authenticated?
There could be a few options to resolving this, but first get the SSL certificate sorted, and if you can provide answers to the above then we can help guide you with the most pertinent answers.