A domain name is pointing to this server but has not been configured correctly.
Or you might have accessed the server by its IP and not by the domain name.
Older operating systems or browsers do not support multiple secure websites (https).>
In the old days, only one secure site could be hosted on one IP (Internet Protocol) address.
Newer browsers and operating systems have changed by adding what is called a Server Name Indication where multiple domains can be securely hosted on the same IP.
If this is the case, there is a good chance your operating system and browser is out of date and you will have to update them.If the problem continues, I recommend you try to access the website through a smart phone or another computer.