"Safari Can't Open the Page".

Please review If Safari doesn't open a page or work as expected on your Mac - Apple Support. There are a few good suggestions in it, but they can be easily overlooked among others that are less useful. Be sure to review all of them.

Often, that message will appear if the Mac's network connection isn't very good. That can be temporary. Try a different network if you can.

In addition to removing website data try clearing Safari's history: Clear your browsing history in Safari on Mac - Apple Support.

For what it's worth I was able to load that site without any trouble, but "can't establish a secure connection" is just that. That's all Safari was able to determine.

Needless to say, if you are using any non-Apple "anti-virus", "cleaning", or "Internet security" products including "banking security", anything at all in that broad category of utterly useless garbage, don't. A lot of things won't work if you do.

Here's an update for you. Using a different browser, I was able to determine the cause:

Um, no. My Mac's clock is correct.

The site is obviously defective. Specifically, it appears to be using a security certificate that's out of date. There is nothing you can do about that. Contact whoever maintains that site and tell them to fix it.

You're welcome.

That site has since been taken down. Presumably, they have been made aware of the problem. Perhaps they're fixing it.

Firefox was a bit less presumptuous, suggesting a different cause just might be a factor:

As it states, there is nothing you can do to resolve it.