Firstly, you will have to install Viber on a mobile device and then register using your email address. After this, you can install Viber on Windows and log in with the email address. This is the only way with this type of application (LINE, TANGO, WeChat, WhatsApp, etc). There is no other way to register from PC since their primary platforms are mobile phones.
I have tried this method and it works normally without errors.