The last backup could not be completed
My iphone 8 don't make backup but i have enough storage, is connected on WIFi,locked on power. I have same issues with my Xr. I tried with other account and tried all things :
Upgrade your iCloud storage plan.
Reduce the size of your backup.
Delete any data that you don't need.
Check that you're connected to Wi-Fi.
Ensure your device is up to date.
Try to back up on another Wi-Fi network.
iPhone 8, iOS 13