The current version of Book Creator requires iOS 15 or higher. If ever you have any issues with the app, we'll always ask you to update iOS and/or Book Creator - the most obvious way of fixing any problems.
On your iPad, go to Settings > General > About and look under Software Version for to see which version you're using.
Alternatively, you could go to Settings > General > Software Update and iOS will check if there's a newer version available for your iPad, as per the screen below.
