A sensor or Cloud Connector that is part of your subscription cannot be deleted.
If you want to terminate the sensor subscription, please visit Managing subscriptions in Studio or contact our customer support team for assistance. Please remember that canceling the subscription does not immediately remove the device from Studio. The device will remain active until its renewal date expires, at which point it will be deactivated and removed from the Studio view.
Is it an emulated sensor or a Cloud Connector?
If it is an emulated device, it can be deleted.