NEXUS - Enrolment on arrival question

Hi everyone, I am a Canadian citizen and I did my CBSA interview (1 out of 2) today. Now I have to do the US interview. The CBSA agent said that I can only do the US part when im flying to the US, but my next flight has a very short layover and I won't have time to do it. I am flying from Ottawa to Toronto, then Toronto to LAX.

I saw on the website that there is a thing called Enrolment on Arrival, where I can complete my second part of the interview (the US one) once I land in an US airport. I asked the agent if I can do that and she said no (?). Has anyone ever done their US interview once you landed in the US? Can I or can I not do it?

please let me know as I will be flying soon.

Thanks