Run npm install after checking out. You might need to restart VSCode after this.
Configure your credentials for either the official or a private server (or both) in the credentials.js file
and when you are ready to deploy, press Ctrl + Shift + B in VSCode and select the desired build action from the list.
The email address it wants is your Steam email address.
If you haven't set a password yet, you'll need to set it here:
http://private.server.host.or.ip:21025/web/change-password
You need to sign in to Steam in your browser before you can change your password (Screeps uses Steam auth tokens behind the scenes).
DO NOT USE YOUR STEAM PASSWORD FOR A SCREEPS PRIVATE SERVER