I haven’t tried code spaces yet but I highly, highly recommend gitpod.io as an alternative. I’ve always loved cloud dev envs and that’s the best one I’ve tried. With a team it’s a serious productivity multiplier for so many reasons, but even alone it’s just so nice to use. Automatic prebuilds on every commit/branch/pr is just chefs kiss.

What is there to learn:

Not much, honestly. You’ll need to know how to dockerize something well enough to create one dockerfile, but we just use one of their provided images and added some tasks in the config file. Since it prebuilds you don’t need to wait opening up a workspace, it’s already done.

Also it’ll help force you to stop relying on and creating little local hacks and workarounds in your dev env out of convenience and laziness (or at least it did for me). That stops so many “works for me” issues. You’ll be a tiny bit annoyed the first time you need to spend an extra few minutes making (for example) something in your test suite work well with “ephemeral” dev envs, and then you’ll be really happy you did when you see that bit get reused by your entire team over and over. It helps your entire app be more encapsulated, deployable, etc. because it has to be for your dev env.

Learn to make use of the features as well, read what they have on offer. Sharing workspace snapshots or an actual workspace instance with your team can be really useful. We create task issues on GitHub for pretty much anything we do and create workspaces right from those. That opens a workspace on a new branch named for the task and is a really nice workflow, especially with the extension that plops a button in github.

Honestly I could go on forever, cloud dev envs are the best, and gitpod is the best one imo.

I wrote a post on how to get up and running with gitpod.io and Elixir. You can read it here.

I also habe a template here: GitHub - benvp/phoenix-gitpod: Sample repository for a Phoenix App running in Gitpod. Also compiles ElixirLS for better IntelliSense support. · GitHub

Stupid question, but is the Phoenix server running?
mix phx.server

Edit:
Or you can try to edit the ip-mapping in config/dev.exs from
http: [ip: {127, 0, 0, 1}, port: 4000]
to
http: [ip: {0, 0, 0, 0}, port: 4000]