1. Sometime your cpanel backup was not complete or got some error during the backup process.
2. After restore the account, the addons domains was not successfully restored.
3. If you try to add the domain using addons domain manager on cpanel, it might occur with an error.
4. It’s because of the domain is on the config files.
5. Please delete all the occurrences domains in those configuration files
6. Now you can add your addons domain.