If the domain name that you are trying to use in cPanel as a parked or an addon contains non-Latin characters (Chinese, Arabic, Cyrillic, etc.) in its name, it needs to be first encoded by using the following tool:
You should then use the converted form of the domain when adding it in cPanel.
If you owned the domain bücher.com its converted alternative would look like this:
In cPanel you need to use xn--bcher-kva.com when adding the domain. When your site visitors type bücher.com, their browsers will automatically convert the name to xn--bcher-kva.com and will then establish a connection to your site.