Android Auto is having a bit of a weird issue lately. It appears that for many that don't have Google Hangouts installed, they are unable to reply to messages (in other messaging apps) with Android Auto. Those that do have Hangouts installed are not having any problems whatsoever with the hands-free replying in Android Auto. However, Google has noted that they are aware of the issue and are working on a fix, so the fix should be available fairly soon. Of course, as usual with software fixes, there's no ETA given for this update.
Luckily, since Android Auto is updated through the app in the app store, which means Google can update everyone's Android Auto without having to go through the car makers, which is a big deal. Let's face it, many people aren't going to want to be bothered with updating the infotainment system in their car, so this is a better course of action. Currently, the way around this issue is to simply install Hangouts, which is a weird fix but it seems to be working. Or you'll just have to live with not being able to reply to messages while you're driving. Which it might be better to wait until you're at your destination to reply anyways, seeing as voice-to-speech still is not perfect, and there are plenty of issues with it in Android Auto.
This issue doesn't seem to be exclusive to Nexus and Pixel devices either, as those with different Samsung and HTC phones are also reporting the issue. So it's a pretty big deal, and it seems to be affecting most messaging apps that support Android Auto. Google is working on the issue though, so it shouldn't be long before it is fixed. Android Auto still isn't a very popular platform for Google, but there are plenty of people using the service right now. Android Auto is available in hundreds of different car models, with more being added almost daily. There are also plenty of head units supporting it, and even your smartphone supports Android Auto, you just need to buy a mount for your car.