While in beta development stage, with every new Naninovel release we're targeting the latest available Unity version. You can always find supported Unity versions per Naninovel release on the GitHub releases page.
In case you need an older Naninovel package, which supports some previous Unity version, please contact the support via a private message with your Asset Store invoice number.
All the engine features are implemented using cross-platform APIs and are expected to be compatible with all the platforms Unity can target.
The following platforms were tested for compatibility: