"Fast TV start" only applies to Roku TVs, not Roku devices connected to a TV. Since Roku has really confused what the term "channel" means, what exactly are you trying to do? You should be able to launch a Roku app with Alexa (although I've never tried it), but if you're trying to change channels on your TV then that's something different. I don't know about the ability to choose a channel within The Roku Channel, but that may be possible. I love Alexa, but the one thing I don't use it for is to control my Roku devices, so others may have better information on what's possible.
You may have seen this, but if not...
How to use Amazon Alexa to control your Roku streaming device | Official Roku Support
Roku Community Streaming Expert
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