Two things:

1.
The book is written to,. or alludes to the following…

Open up an operating system shell and navigate to the parent directory for your project. Then, type:
mix phx.new pento --live
…
[pp_liveview] ➔ mix phx.new pento --live * creating pento/config/config.exs

• creating pento/config/dev.exs
• creating pento/config/prod.exs
  …
  Fetch and install dependencies? [Yn] Y
• running mix deps.get
  …
  The mix phx.new command runs the Phoenix installer for a standard Phoenix project, and the --live switch adds the library dependencies, configuration, and assets we’ll need to build live views.

As we work through this book, we’ll point out the bits that this --live switch adds to our project, and we’ll examine the directory structure in detail over time. For now, know that backend code goes in the lib/pento directory, the web- based assets like .css and .js files go in assets, and the web-based code all goes in the lib/pento_web directory.
Create The Database and Run The Server
At the bottom of the installation output, you’ll find a few extra instructions that look something like this:
…
We are almost there! The following steps are missing:
$ cd pento
$ mix deps.get
$ cd assets && npm install && node node_modules/webpack/bin/webpack.js
–mode development
Then configure your database in config/dev.exs and run:
$ mix ecto.create

I think too many assumptions are made at this step, especially if you are rusty. mix ecto.create errors out. You need to be specific about what those configs need to be set at in config/dev.exs. Also which version of PostgreSQL? A suggestion GUI-driven version probably would be a good idea to help standardize this.

2.
The book above (#1) says one thing but the printout I get from running mix phx.new pento --live produced s slightly different set of instructions at the end of the project generation.

(base) bradhutchins@MacBook-Pro Programming-Phoenix-LiveView % mix phx.new pento --live

• creating pento/config/config.exs
• creating pento/config/dev.exs
• creating pento/config/prod.exs
• creating pento/config/prod.secret.exs
• creating pento/config/test.exs
• creating pento/lib/pento/application.ex
• creating pento/lib/pento.ex
• creating pento/lib/pento_web/channels/user_socket.ex
• creating pento/lib/pento_web/views/error_helpers.ex
• creating pento/lib/pento_web/views/error_view.ex
• creating pento/lib/pento_web/endpoint.ex
• creating pento/lib/pento_web/router.ex
• creating pento/lib/pento_web/telemetry.ex
• creating pento/lib/pento_web.ex
• creating pento/mix.exs
• creating pento/README.md
• creating pento/.formatter.exs
• creating pento/.gitignore
• creating pento/test/support/channel_case.ex
• creating pento/test/support/conn_case.ex
• creating pento/test/test_helper.exs
• creating pento/test/pento_web/views/error_view_test.exs
• creating pento/lib/pento/repo.ex
• creating pento/priv/repo/migrations/.formatter.exs
• creating pento/priv/repo/seeds.exs
• creating pento/test/support/data_case.ex
• creating pento/lib/pento_web/templates/layout/root.html.leex
• creating pento/lib/pento_web/templates/layout/app.html.eex
• creating pento/lib/pento_web/templates/layout/live.html.leex
• creating pento/lib/pento_web/views/layout_view.ex
• creating pento/lib/pento_web/live/page_live.ex
• creating pento/lib/pento_web/live/page_live.html.leex
• creating pento/test/pento_web/views/layout_view_test.exs
• creating pento/test/pento_web/live/page_live_test.exs
• creating pento/lib/pento_web/gettext.ex
• creating pento/priv/gettext/en/LC_MESSAGES/errors.po
• creating pento/priv/gettext/errors.pot
• creating pento/assets/webpack.config.js
• creating pento/assets/.babelrc
• creating pento/assets/js/app.js
• creating pento/assets/css/app.scss
• creating pento/assets/package.json
• creating pento/assets/static/favicon.ico
• creating pento/assets/css/phoenix.css
• creating pento/assets/static/images/phoenix.png
• creating pento/assets/static/robots.txt

Fetch and install dependencies? [Yn] Y

• running mix deps.get
• running mix deps.compile
• running cd assets && npm install && node node_modules/webpack/bin/webpack.js --mode development

We are almost there! The following steps are missing:

$ cd pento

Then configure your database in config/dev.exs and run:

$ mix ecto.create

Start your Phoenix app with:

$ mix phx.server

You can also run your app inside IEx (Interactive Elixir) as:

$ iex -S mix phx.server