Hi,
I was facing the same error message lately. To get rid of it I just deleted the end action of the last existing chapter in my timeline. I also thought that it was necessary to have endactions for any single chapter until I found this thread here. In future project I will play a little with the options.
While I was learning the functionality of Encore in the last days I among other things watched the tutorials from Jeff Sengstack on youtube. It helped me a lot but I'm still not aware of the advantages of chapter playlists. Within the tutorials it says that with CP's you can create the project that way that if you click on a single scene it will not play all the following scenes as well. It will stop after that one scene and go back to the menu where the following scene is highlighted. But the same effect do I have when I put the "override action" of a chapter point to the follower of that watched scene. So maybe there are other advantages of chapter playlists that I don't see. Maybe someone can tell me. Right now I don't think that not avoiding overrides is such a big deal and costs such a lot of time compared with CP's.
In my next (2nd) profect I will test the chapter playlist to see what it means to have them. ^^