Sometimes you are trying to log into your cPanel account via your domain – https://your_domain:2083 or your_domain/cpanel however you cannot open that URL or cannot log in even though you are using the correct username and password.
These issues usually happen when the DNS changes are not propagated completely throughout the Internet. So until they are, you can access your cPanel account at: https://your_LinuxVPS_IP:2083/. Or if you need to access WHM, visit: https://your_LinuxVPS_IP:2087.
There is another mistake that people usually do and that is to try and log into a cPanel account using [email protected] as username when in fact they need to use the actual cPanel username which is not an email address. In WHM you can see all the created cPanel accounts and their usernames by navigating to: Home »Account Information »List Accounts.