Q: I am not able to delete my previous phone's backup from icloud

See this -- About backups in iCloud and iTunes - Apple Support

If you have a Mac computer, System Preferences/iCloud and click on Manage. Delete the backup there.

If you do not have a computer, try to wait out for the small activity wheel to stop circling and then attempt to delete backups.

If still unable try to sign out and sign back in to icloud and then again to wait for the wheel to stop and then delete those.