您的Roles型号没有与GetRolesResponse中的Features匹配的相应List，因此Features将为null。
你能改变RoleFeatures和FeaturesDTO的模型结构，使两者匹配吗？如果你可以的话，它将非常简单。下面是一个例子：
型号：

public class Roles
{
    [Key]
    public long RoleIdPk { get; set; }
    public string RoleName { get; set; }
    public string RoleDescription { get; set; }
    public int CreatedBy { get; set; }
    public List<RoleFeatures> RoleFeatures { get; set; }
}

public class Features
{
    [Key]
    public long FeatureIDPk { get; set; }
    public string FeatureName { get; set; }
    public string FeatureDescription { get; set; }
    public int CreatedBy { get; set; }
}
public class RoleFeatures
{
    public long RoleIDFk { get; set; }
    [Key]
    public long FeatureIDFk { get; set; }
    public string FeatureName { get; set; }
    public string ViewPermission { get; set; }
    public string addPermission { get; set; }
    public string EditPermission{ get; set; }
    public string DeletePermission { get; set; }
    [ForeignKey("RoleIDFk")]
    [JsonIgnore]
    public Roles Roles { get; set; }
}

public class GetRolesResponse
{
    public long RoleId { get; set; }
    public string RoleName { get; set; }
    public List<FeaturesDTO> Features { get; set; }
}
public class FeaturesDTO
{
    public long FeatureId { get; set; }
    public string FeatureName { get; set; }
    public string ViewPerm { get; set; }
    public string AddPerm { get; set; }
    public string EditPerm { get; set; }
    public string DeletePerm { get; set; }
}

公司简介：

public class MyProfile : Profile
{
    public MyProfile()
    {
        CreateMap<Roles, GetRolesResponse>()
            .ForMember(dest =>
                dest.RoleId,
                opt => opt.MapFrom(src => src.RoleIdPk))
            .ForMember(dest =>
                dest.Features,
                opt => opt.MapFrom(src => src.RoleFeatures));

        CreateMap<RoleFeatures, FeaturesDTO>()
            .ForMember(dest =>
                dest.FeatureId,
                opt => opt.MapFrom(src => src.FeatureIDFk))
            .ForMember(dest =>
                dest.ViewPerm,
                opt => opt.MapFrom(src => src.ViewPermission))
            .ForMember(dest =>
                dest.EditPerm,
                opt => opt.MapFrom(src => src.EditPermission))
            .ForMember(dest =>
                dest.AddPerm,
                opt => opt.MapFrom(src => src.addPermission))
            .ForMember(dest =>
                dest.DeletePerm,
                opt => opt.MapFrom(src => src.DeletePermission));
    }
}

控制器：

public async Task<List<GetRolesResponse>> IndexAsync()
{
    List<Roles> roles = await _context.Roles.Include(r => r.RoleFeatures).ToListAsync();
    var roles1 = _mapper.Map<List<GetRolesResponse>>(roles);
    return roles1;
}

测试结果：