The last backup could not be completed

Try to reset your device. This won't delete your data stored on this device.

• Press and hold the Sleep/Wake button
• Press and hold on the Home button
• Keep holding both buttons until the display turns off and back on with Apple logo on it.

Alternatively, you may go to Settings - General - Reset - Reset All Settings

Take a look at this article:

Get help backing up your device in iCloud - Apple Support

If the steps offered by Apple don't work, try contacting Apple Support and asking for help:

https://www.apple.com/support/contact/

I have tried restarting my phone and I am still not able to backup.

I tried the above steps and they didn't help. What I finally did to get this working was I went to Settings -> iCloud -> Backups, then selected my iPhone backup and deleted it (which also turns off backups to iCloud). Then reset my phone (hold down the home button and power button for about 5 seconds until the screen goes black, then release them and the phone will restart. Nothing is lost by doing this). Then went back to Settings -> iCloud -> Backups to turn backups back on. Then did Backup Now and it worked!

BTW: Since you are going to delete your iCloud backup, I'd advise plugging your phone into the computer, going into iTunes and doing a manual backup to your computer just so you have a backup available during this process.

Here's the steps I was just given by an Apple support person. But, Apple is aware of the problem. It will be fixed in next update. So, you may want to wait, and backup to iTunes on your computer

1. Back up the device to iTunes.

2. Perform a recovery mode restore - resource included.

3. During the setup assistant, choose to restore from the iTunes backup.

4. After the restore has completed, go to Settings > iCloud > Storage > Manage Storage > [Device Name] and delete any existing backups for the problematic device. Note that when you first go to the Settings > iCloud > Storage > Manage Storage screen, then tap the device name (this device), you must wait for the screen to load completely (and wait for the spinning pinwheel in "Backup Options" section to go away) before deleting the backup).

5. Go to Settings > iCloud > Backup and turn iCloud Backup off.

6. Wait about 2 minutes and turn iCloud Backup back on.

7. Try to backup to iCloud again.

If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support