When I try to back up my iphone onto icloud it won't do it and I get a message saying "The last backup could not be completed." What am I doing wrong and why won't it backup?

HI there FLYBOY1984,

It sounds like you are unable to complete an iCloud backup for some reason. I would start by turning off iCloud backups in your settings:

In iOS 8 or later, tap Settings > iCloud > Backup.

From: How to back up your devices using iCloud or iTunes

https://support.apple.com/en-us/HT203977

Then quit the Settings app:

Force an app to close in iOS
https://support.apple.com/en-us/HT201330

1. Press the Home button two times quickly. You'll see small previews of your recently used apps.
2. Swipe left to find the app you want to close.
3. Swipe up on the app's preview to close it.

When you have done that restart the device and test the issue again:

iOS: Turning off and on (restarting) and resetting
http://support.apple.com/en-us/HT201559

Then turn iCloud backup back on and try again. That should get it going again. Thank you for using Apple Support Communities!