If you see that message pop up when you're trying to download GoodNotes 5, it likely means that your device does not meet the system requirements of GoodNotes 5, which needs iOS 12 or later.
Otherwise, please do the following:
1. Open the Settings app > Apple ID > Media & Purchases > Sign Out.
2. Visit GoodNotes App Store page and choose Get.
3. Sign in to your Apple ID account and continue with the purchase process.