Why am I seeing this message on my phone?
iPhone 11 Pro Max
Did you make an iCloud backup recently?
Check Settings > [your name] > iCloud > iCloud Backup: ON?
New feature: Apple provide free temporary space (more than what you have in iCloud to create a backup) for moving data onto a new iPhone. As stated this temporary storage will be deleted in 20 days.
Learn how to back up your device using iCloud or how to manage iCloud storage and delete backups that you no longer need.