In my case, I had created the map, but was missing the ReverseMap function. Adding it got rid of the error.
private static void RegisterServices(ContainerBuilder bldr)
{
var config = new MapperConfiguration(cfg =>
{
cfg.AddProfile(new CampMappingProfile());
});
...
}
public CampMappingProfile()
{
CreateMap<Talk, TalkModel>().ReverseMap();
...
}