You can use the "erase all content and settings" to completely "wipe" the new MBP.
Then you can start over, it will be the same as if you just took it out of the box for the first time.
How to do this:
a. boot to the finder.
b. you will need a usable account with an administrative password
c. open system preferences
d. in the system preferences menu (up in the menu bar), choose "erase all content and settings".
e. this invokes the "erase assistant" -- just follow through
f. I believe when done, this should return you to the original startup screen (choose your language).
Something else:
WHAT method were you using with migration assistant?
Actually, for a Mac right-out-of-the-box, you SHOULDN'T be using migration assistant.
You SHOULD be using setup assistant, which runs as the initial setup app.
Were you trying to migrate from a backup drive?
or
Were you trying to migrate using a connection to the older Mac?
My advice is to NOT use a "live connection" to the older Mac.
Rather, use a backup on an external drive.
Just before you begin setup, run the backup "one last time" so it's up-to-date.
I also do not recommend time machine for this.
I recommend using a cloned backup created with either CarbonCopyCloner or SuperDuper instead.
Both of these apps are FREE to use for 30 days, so creating one will cost you nothing.
So, to repeat:
- create a backup on an external drive and have it ready BEFORE you begin the setup
- use erase assistant to "wipe clean" the MBP
- as soon as you see the initial setup screen, CONNECT THE BACKUP
- begin setup
- setup assistant will ask if you wish to migrate from another drive -- YES.
- "point the way" to the external backup and give SA time to digest everything
- SA will give you a "checklist" of things to migrate -- I'd suggest "everything"
- let SA do its thing
- when done, you should see your old login screen, just as before.