You can easily backup and configure the iPhone and iPad to iCloud every evening when connected to Wi-Fi. Sometimes you may get an error message saying that "The last backup could not be completed". You may get this backup failed message when you try to complete an automatic or manual iCloud backup from an iOS device. To solve this issue, you can reboot the iOS device and try again or delete an old backups, reset network settings and then try again.
1. Reboot the iOS Device and Try Again:
This is an easy and most reliable method to try before anything else. It is simply rebooting the iOS device and trying to backup photos and videos to iCloud again when it boots back again. to do so, hold down the "Home" button and "Power" button simultaneously until you see the Apple logo. Let your device to boot up again as usual and make sure that your device is connected to a Wi-Fi network. Return to "Settings" on your iOS device and tap "iCloud" and then tap "Backup" option and then tap "Back Up Now" button. You need to be sure that you are on best Wi-Fi network connection, otherwise you will cause this issue.
2. Delete Old iCloud Backups, Reset Network Settings and Try Again:
If you are getting "the last backup could not be completed" issue, then you can delete your old iCloud backups and reset network settings and then try to backup again. To do so, first you need to backup your iPhone or iPad to a computer, before deleting old iCloud backups. Once you have backed up to computer, open the "Settings" app on your iOS device and tap "iCloud" and "Storage" and then tap "Manage Storage". Now, locate the old iCloud backup and delete it from iCloud and then clear out the network settings in iOS Settings by going to "Reset" and then "Reset Network Settings". Once you remove your Wi-Fi networks, join it again from iOS device. Now return to "Settings and tap "iCloud and "Backup" and select "Back up Now" option.