I was within my 60 days.
I noticed that I could see my reservation in the
MDE system but couldn't make FP+ selections. My APs were in the system, too, but still couldn't make FP+ selections.
Then it dawned on me that my APs that were in they system were set to expire prior to the trip.

I put in my AP renewal voucher number and, viola, I could THEN put in my FP+ selections.
If you are within your 60days, make sure to input your ticket information and you should then be able to make selections.