My understanding is that, like many security systems, STHM, does not allow automating the dismissal of an alarm. Once it gets to that point, a person must look at it in the app and manually dismiss it from there. You can’t dismiss it with the built-in features like an automation, or a rule, which is why you can’t dismiss it from a third-party system either.
You can disarm, but only if you can do so before the alert is sent
See the following discussion from a few months ago. Some people have built workarounds, but you basically have to replace the STHM functioning with your own routines.
How to disarm and dismiss in smartthings home monitor (STHM)?