I had been trying unsuccessfully since the live date to merge my SPG account with my Marriott account.
The system simply wouldn't recognize my SPG account number, and finally I gave up and called the Platinum helpline. They were unable to explain but promised to call me back. The called me back this afternoon and the problem is that they AUTOMATICALLY CHANGED SPG ACCOUNT NUMBERS that were duplicated as Marriott numbers. They had to let me know the new number my SPG account had been assigned, then I had to initiate a password change on that new account number. At that point I was able to log into my Marriott account and initiate the merger using the new SPG number and my newly changed password.
Hope this helps if anyone is seeing the merger fail as it doesn't recognize your SPG login.