I’m not sure why it didn’t occur in the installation process.I’m on DigitalOcean and I created a few Droplets already and had the same issue each time.
You should have gotten an error if it failed. Are you sure the installation completed? It shows steps and stages, so you can be sure it completes (i.e. you’ll see “Stage 3 of 3” and “[23/23]” (or similar, it’ll be different depending on which mode you’re using) and none of them with a red X?
If Postfix failed, I’d guess other steps failed, too, or didn’t run at all.