For install the app you are tring to install you need a working domain.
So in the choose domain have you selected the domain?
You can reach this domain by put the link in the browser?
I think you have not setup correctly domain. Where is registered your domain? Because from where is registered you can manage DNS and create A record of the domain who should point to AN IP. Your server IP. After that you need wait DNS propagation 24 to 48 h