The custom command feature no longer works with SHM or Speaker Notify with Sound because they broke the custom message functionality in those SmartApps.
You can still use the device as an alarm or switch in those SmartApps, but if you want to be able to use features like beep, you will need to use a community developed SmartApp like either Rule Machine or CoRE.
The developer of Rule Machine doesn’t allow new installs so unless you already had it installed, you will have to use CoRE instead.