I have tried adding it to a html stack and also using stack4stacks embed stack but in each case the frame size shows but with no content and obviously no border. The https link in the iframe works on its own just fine. I am adding it to a https website so there is no conflict between http and https in the iframe. I have also tried formatting the code but that didn’t make any difference.
I suspect the site you’re trying to iframe may be blocking it purposefully via meta tags in their site. I could be misreading that though. That said, if this iframe is a service the website you’re using provides, you’ll want to contact them directly and ask them to assist.
I was not sure what was going on but the company who make the booking system have fixed it after many emails. Here it is working: elevatebodyclinic.com.