can't add a C++ class to project

Hello!

It seems something is wrong with the "File and Code Templates" settings. 

You can try restoring IDE settings - File | Manage IDE Settings | Restore Default Settings > Restore and Restart. The IDE will be restarted with the default configuration. When CLion restores the default IDE settings, it creates a backup directory with your configuration, so you will be able to apply the IDE settings from a backup if needed. More info - https://www.jetbrains.com/help/clion/configuring-project-and-ide-settings.html#restore-defaults