Remember that this issue does not change master. 2.8 and 2.7 only. You will need a publicly accessible site. You can make your development site available on the net using ngrok (google for it). If using ngrok do not password protect the subdomain (this will make sense once you see ngrok's docs) as moodle.org/net needs to be able to see your site. If your site is already registered, unregister it. Go to site admin > registration. There should be a notification at the top saying your site is not registered. Register your site. Return to site admin > registration and check that it now says you are registered at the top.