Hi there
I also struggled at first, but it's because I added the wrong number of applicants. It's your spouse + you (+ whomever else is on the application with you, like your children). So in my case, it had to be 2 and not 1.
And I used the identification option that required my name, surname, etc and not the one that required my application number and UCI because none of those are on the application I mailed in. And the instructions say to copy the information exactly as it is on your application.
Good luck!