How come I cannot delete an old iCloud backup?

I have an iPhone 7 and just upgraded to an iPhone 8. When I tried deleting the iCloud backup of my iPhone 7 I get an error, "Cannot Delete Backup - This backup cannot be deleted at this time." and with the only options of OK or Try Again. I've tried deleting from 3 methods: directly from the iPhone 7, iphone 8 and via the Windows iCloud; all options says the same error. Please help as it is occupying 2GB of my iCloud storage.