Great! You see 50 GB of storage in iCloud and you likely have a significant amount unused.
Two things to try:
- Restart the device. That should force a new "connection" to iCloud and eliminate the error of iCloud being full
- Sign out/in to iCloud. If the restart doesn't work, sign out and back into your Apple ID.
If your device is low on space, you may need to delete pictures, videos, apps, data as well. Generally, you'll want to try to keep at least 5 Gb free and possibly up to 10 GB free on your device at all times so syncing and other functions work well.
It's also possible that your next iCloud backup (if you're backing up to iCloud), exceeds 50 GB. That would be somewhat unusual but not unheard of if you don't use iCloud photos, iCloud Drive, etc.