Ulysses beta can only be used if an App Store subscription is active. Otherwise, your contents are read-only and you cannot edit or create sheets anymore. You can still export your contents to continue editing them in other applications.
To begin a subscription, you need to uninstall the Beta first and install Ulysses from the App Store. From there, you can begin a subscription. Once you have activated it, you can re-install the beta via TestFlight.