To do this you need to either create a "virtual" thermostat device, or make use of the Q&A Skill, so that Alexa knows what you are asking it for.
I see your flow comes from this post trying-to-get-alexa-to-read-out-a-temperature
Did you read it all ?
It provides solutions and also links to another thread about the Q&A skill pure-fun-with-alexa-and-alexa-skills