If this is the first time opening the Luna Playable plugin, you will be presented with an interface requiring your Luna Playground login credentials.
Log in to your Luna Playground account
(If you don't have credentials, you should reach out to your account admin or sign up here.)
Once logged in, the only mandatory change will be to select the scenes to include in the export, and your startup scene (this will be set to default if you have only one scene). To do this, you can navigate to the Settings section, and select the Basic tab.
You can read more about this here.