This website does notsupport connecting securely over HTT

Ok... and...?

Are you asking how to make the site use HTTPS? That's up to the site developer.

Are you asking why Safari is warning you about it? Some people consider this a security issue and if they expect the site to be using HTTPS then they'd want to know if the site is unsecured for some reason (this could indicate some kind of malicious user impersonating the site you're trying to access, for example).

If you don't care and want to use the unsecured site, there's an option in Safari Settings related to 'Non-secure Connection warnings'. Turn that off and you shouldn't see the message (although you now have no way to know if the site you're accessing is secure or not.