Galaxy Watch Designer / Galaxy Watch Studio for Tizen Watches are no longer supported I’ve asked them to place a notice about this.
For the past 3 years Samsung has use Wear OS for the platform you can but a Galaxy Watch4 for a very good price and it runs the same operating system as the latest Galaxy Watch6 does. Samsung has Watch Face Studio a free tool for the Wear based watches.
See this thread for a way to install the tpk but it is a lot of extra work.
Ron
Samsung Developer Relations