I've been selecting FastPass+ reservations each evening at 9 p.m. for an upcoming WDW trip. Last night, the My Disney Experience app told me I couldn't make any more FastPass selections because I had "reached the maximum amount of FastPass+ planning days". I currently have seven days of Fastpasses, but my WDW tickets are for nine days.
I contacted Walt Disney Travel and spoke with a My Disney Experience technician. There is a limit of seven days of FastPass reservations, regardless of the number of days on your tickets.
I can't make more FP selections until after the first day's FastPasses expire.