How to map nested child object properties in Automapper

This should work. Use ForPath instead of ForMember

CreateMap<Article, ArticleModel>()
    .ForMember(dest => dest.BaseContentItem, opts => opts.MapFrom(src => src.BaseContentItem))
    .ForPath(dest => dest.BaseContentItem.TopicTag, opts => opts.MapFrom(src => src.BaseContentItem.TopicTag))
    .ForMember(dest => dest.MainImage, opts => opts.MapFrom(src => src.MainImage))
    .ReverseMap();