What to do if iCloud is not working on iPhone?

Make sure you have got the right path to iCloud. Check as below steps:
Step 1: Navigate to "Settings > iCloud". Log in iCloud with your Apple ID.
Step 2: Tab "iCloud Backup". Make sure the button is on green.
Step 3: Check if you have enough available space in iCloud for the backup. Normally, you will get 5GB iCloud storage for free. If you need more, you have to buy from Apple.
Step 4: After checking steps below, if you found that iCloud backup is slow, you can refer to this tutorial for your reference: How Long Does it Take to Backup iPhone to iCloud. If steps above don't work, you have to turn to iTunes or other third party programs.

Delete the app and reinstall it

• iCloud Backup requires a Wi-Fi internet connection.
• If you have a lot of data iCloud backups and restores may take time.
• iCloud Backup can fail if you do not have at least 50 MB of available disk space on your device.
• Automatic backups happen when your device is plugged in with the screen locked, and also connected to Wi-Fi.
• While you are downloading apps or songs, backups will not occur. It's a good idea to let it finish.