This arises from 3 possible problems:
1. The airplane can't physically complete a segment. This often happens in constant throttle type segments. As an obvious example, there's no airplane that climb at 0% throttle at a constant airspeed on a standard day. Check to make sure you're asking it to fly a reasonable mission.
2. There is a value in either your vehicle or mission that is not set. A value of 0 can lead to NaN's later. Check for obvious things that may be set to 0, like a wing area. You can also take a look at the results and see if anything in the dictionaries is NaN. Specifically, it's worth taking a look at the residual values.
3. Try different unknown values. The initial conditions are defaulted, but you can change those as well. Usually eVTOLs are the most finicky to these settings especially in throttle, voltage, and propeller CP. Sometimes I start from the value the solver gave up on. Other times I change it a lot.
This error unfortunately happens more than I would like. But with a little detective work you can figure out what's going on.