Which sensor are you trying to get Alexa to see? Alexa can’t run an Automation but it can operate your SmartPlug.
If you’re using the Classic app there is something called a Routine (which was basically an Automation + Scene combined into one) which could be run by Alexa.
With the new App Alexa can execute a Scene (not an Automation), so ideally you want to group your actions into a Scene and then have the Automation execute the Scene and Alexa can also execute that Scene in SmartThings as explained on this link.
If you’re looking for a more comprehensive way to convert your multisensor + smartplug into a Virtual garage door, check out this app: [RELEASE] Virtual Garage Door Opener/Controller with Relays and Contact/Tilt/Door Sensors