az extension add --name azure-cli-iot-ext is old version, use the newest version of the Azure IoT extension, called azure-iot

 https://docs.microsoft.com/en-us/azure/iot-hub/iot-hub-device-management-iot-extension-azure-cli-2-0

 the newest version of the Azure IoT extension, called azure-iot. The legacy version is called azure-cli-iot-ext.You should only have one version installed at a time. You can use the command az extension list to validate the currently installed extensions.

Use az extension remove --name azure-cli-iot-ext to remove the legacy version of the extension.

Use az extension add --name azure-iot to add the new version of the extension.

To see what extensions you have installed, use az extension list.