I have an issue with my iCloud backup .

I don’t know if this is a new issue for you, or if you just recently upgraded your iCloud storage, but you can go through this and see if anything in here helps. Frankly, I don’t think that anything will be applicable. But it can’t hurt to take a look.

If you can't back up to iCloud - Apple Support

If nothing in there helps, there are links that you can use to contact Apple Support for help.

I don’t know if this will fix it, but it won’t hurt to try. Turn off WiFi on the phone and turn off iCloud backups in the settings. Restart your phone, turn on WiFi and the iCloud backups again and then try to backup again.