If you're running out of space on your Galaxy Note 9's internal storage you might need to clear up some space for the Android Pie update.
The Galaxy Note 9 Android Pie update will require a download somewhere between 1.5GB and 2GB. If you don't have that kind of storage available you'll need to delete files in order to start the download/installation process.
If you're near the threshold, use this time to get rid of files (photos, music, etc) you no longer need. Think of it as early spring cleaning.
You can use your Galaxy Note 9's Device maintenance feature, located in your Settings, to help narrow down the list of files to delete.
If you cleanup your storage ahead of time you'll be ready to download Android Pie as soon as it arrives. You might even seen a bump to your device's overall speed as well.