It looks like your iPad or Books app is not running the latest available version.
Before trying to download the user manual, go to the Settings app on your iPad, select General, and go to Software Update. See if there‘s an update, and if so, install it.
Or maybe the app needs an update. The app‘s current name is Books (or Apple Books). Apple doesn‘t use iBooks any more.
To find out whether or not there‘s an update for the app, go to the App Store, and search for Apple Books (developer: Apple). If there‘s an update, install it.