We apologize for the inconvenience. Please check the following.
First, make sure the Apple Account you are signed in with on the device is the same one you used when creating the backup.
Then restart the device and check again.
iCloud sometimes has temporary glitches, and restarting the device often makes the backup appear.
If that does not help, open the device's "Settings" app, tap your name at the top (your Apple Account), then tap "iCloud", and turn off "iCloud Drive".
Then turn it back on, wait 5 minutes, reopen the app, and check the iCloud Backup screen. iCloud Drive occasionally has glitches, and this procedure resolves them.
We apologize for the trouble.
If it still does not work, please tap the "Contact Us" button at the top right and describe the situation in detail.
Thank you.