Why can't I find the Google voice channel
I don't believe there's any such channel. Some Roku remotes offer voice control, but it has nothing to do with Google/Alexa. If your Roku device doesn't have a voice remote, there's no way to add voice control. If your Roku did not come with a voice remote, some models support adding one. But some devices are IR control only, so the Voice Remote isn't an option at all.