mas does not currently support signing in using an Apple ID with 2FA enabled. Tested with mas 1.4.2 on macOS 10.11, the experience is as follows:

Steps

1. Use signin command with only a user argument
2. mas asks for a password
3. Enter correct password

Result

• mas immediately gives an error (below)
• meanwhile the Apple 2FA dialog appears on another Mac and notification on iPhone of same user

$ mas signin 2fa-user@icloud.com
**==>** **Signing in to Apple ID: 2fa-user@icloud.com**
Password: 
~Error:~ Sign in failed: The operation couldn’t be completed. (mas.MASError error 0.)

Need to add support to wait for the 2FA process completes. Timeout? User should always be able to cancel with CTRL+C.