You may encounter this error when deleting a device:
This is because the device you are trying to delete is still actively using the Internet whilst connected to the router.
In order to delete it, disconnect the device from the router then do a reboot from Settings > Miscellaneous. Then go back to the device manager and delete the device.
Please note if the you do not disconnect the device before rebooting it may automatically connect again and show up in the device manager.