Page 15 shows these imports for the conversion of application.js to application.ts

import * as ActiveStorage from "@rails/activestorage"
import Rails from "@rails/ujs"
import * as Channels from "channels"
import Turbolinks from "turbolinks"

However I had to change the Channels import to the following to get a build:

// from @rails/actioncable not channels
import * as Channels from "@rails/actioncable"

Environment: Ruby 2.7.1 Rails 6.0.3.2 Ubuntu 20 LTS