Apple made several changes with iOS13 that make the current version of the Nuki App unusable on iOS13 devices (especially new installations of the App). We’re working on gradually restoring compatibility, but due to the severity of the changes to the operating system this will take longer than usual.
Updates will roll into upcoming beta releases of the Nuki iOS App. Make sure to sign up for beta. Any changes to iOS13 compatibility will be mentioned in the TestFlight release notes.
Full compatibility will only be available when Apple releases iOS13 (most likely by end of September). Please keep in mind that iOS13 is currently only a beta version of the OS which Apple recommends to not use on production devices.