Hello. The error message explains why you can't register your site. It is because your site is not online and Moodle needs to be able to communicate with it in order to register it - and it cannot communicate with a site that is not online.
If you only want to use Moodle for testing things out then you don't need to put your site online and you don't need to register it. If you do want your site online to use it with students then follow instructions in Installing Moodle or you could simply sign up for a free MoodleCloud site which is already set up online for you.