The cPanel control panel does not support IDN domains (including Cyrillic ones) written in any non-Latin characters.
However, adding IDN domains to the control panel is possible using Punycode. You can get Punycode in the following way (for example, домен.com.ua):
- go to the DIG.UA website,
- enter домен.com.ua domain in the search bar,
- copy the Punycode that will appear in the "domain" field (for the domain домен.com.ua it looks like this: xn--d1acufc.com.ua)
Use the copied value while adding the domain name in the cPanel hosting panel.