There are several known bugs that can result in the pickup game showing "scheduled successfully" on the scrimmages page but not actually scheduling.
In my experience, I have the worst troubles when I try to schedule pickup games less than 65 minutes in the future, or within 15 minutes of another game on my schedule. But it also depends on not being too close to other games on the pickup bot's schedule, which you can't see, so you have to kind of trial and error with times sometimes.