I don’t believe this will work if you are using the default device handler for garage door openers. This is considered a security risk and not made available as a switch by default. You need to install a custom device handler that exposes the garage door opener as a controllable switch and then it will be seen by the Alexa smartApp. See below for the handler I use (NOT FREE):
Here is a free one that I have not used or tested but should be worth a shot:
For step by steps…check out this wiki: http://thingsthataresmart.wiki/
The FAQ sections has guides on creating custom device handlers, etc