He doesn’t use Litchi.
He’s trying to develop his own app using DJI Mobile SDK v4.16.2 and is still using/modifying a Github sample app.
He posted the same question in the MavicPilots Forum for some reason:
My guess is that this specific SDK version has a maximum waypoint corner radius limit of 500m and/or a maximum 3D distance limit of 500m between 2 consecutive waypoints and/or between the home point and WP1, specific for the Phantom 3 Standard and he exceeds one of them.