Regular backups of the devices are extremely important and should be done in any case so as not to lose more data than necessary in the event of a system crash (or loss). With its iCloud backups, Apple already offers a good automated service. But here and there it sometimes comes to problems, so that the backup can not be completed.
Solution 1: The restart and a new attempt
Before you resort to more complicated means, you should Try it with a forced reboot. This will already fix many bugs.
Hold down the Home button and the Power button at the same time until the Apple logo appears on your screen. The restart works only up to the iPhone 6s. Here's how it works on other models → iPhone 7, iPhone 8, iPhone X
Wait for your device to start up and check the Wi-Fi connection in the Settings app Your network. Then, in the Settings app, tap your Apple ID> iCloud> iCloud Backup> Create Backup Now to start the process again.
Solution 2: Backup delete via iTunes & iCloud backups
If the first solution did not work despite a very good Wi-Fi connection, you have to resort to a more complex method. Simply delete old iCloud backups after creating a new backup with iTunes.
Connect your iOS device to your Mac and open iTunes. Create a local backup with iTunes. After the backup is created, pick up your iOS device and open the Settings app. Navigate to your Apple ID> iCloud> Manage Storage, and then open the backup data for your device under Backup. Tap "Delete Backup."
Return to the settings. Open General> Reset, and then tap Network Settings. After entering your unlock code, you will lose all your saved Wi-Fi information so you will need to enter it again. Re-enter your Wi-Fi data and start a manual iCloud backup under your Apple ID in iCloud> Backup.