You may receive this message while preparing the devices using Apple configurator.
Please login to the Hexnode portal to verify the URL. You may also need to choose a default user too. Please navigate to Admin-> Configurator Enrollment-> Choose the Default User->Save the Default user. Once this is done, try enrolling the devices again.
I was trying to enroll my iPdas using AC2. However, I receive this: An unexpected error has occurred: The device returned an unexpected status. (CommandFormatError)[com.apple.configurator.MobileDeviceKit.error]
The possible cause of this error is that you may be trying to update iOS 11, along while trying to enroll the devices in DEP, using Apple Configurator.
Please complete the OS update process and then try to add it to DEP.