Wow, that's odd! I did have a problem with it once, where I e-mailed the Ridemax people. I was getting something similar, but in my case, it was because I didn't have at least 3 rides listed. They told me I had to have 3 rides before it would make a plan. That doesn't sound like your problem.
Are you trying to make separate itineraries for morning and evening touring, as a previous thread talked about? So will you have Fast Passes collected from that morning? Because Ridemax won't be allowing for that. It might assume that, by 5 p.m., there won't be any FPs left for Splash, Space or Indy, and possibly the other FP attractions. So if you have more than 3 headliners, they will likely have long lines (60-90 mins), and Ridemax will tell you there's not enough time to fit them in, because it's assuming you don't have FP for them.
Also be sure you have the "ending time" correct. If it's on the default, I believe it says something like "I will get in line for my last ride at 8:10 p.m." which assumes you want to see the fireworks. If that wasn't in the plan, you should have that time say "11:50" or whatever.
However... If that's your first day and you get to the parks at 5 p.m., any FPs you acquire will probably have a very late return time. You might want to wait, if you can, for the next day or two, to do the headliner FP rides by collecting the FPs in the morning. Do other touring stuff on Thurs. night, explore, watch
Disneyland: The First 50 Years, shop for souvenirs, ride any smaller rides without big lines (like maybe Pinocchio, Snow White, Toad, HM, PotC, Winnie) see the fireworks and Fantasmic. Maybe get in line for Nemo last thing at quarter to 12, and get that puppy knocked off your to-do list.
Don't know if that makes any sense or helps at all. Hope you can get it all figured out!
