Tente usar a opção target_group_arns
no ASG.
resource "aws_autoscaling_group" "asg" {
name = "test"
vpc_zone_identifier = ["${data.aws_subnet.sn-app-1.id}","${data.aws_subnet.sn-app-2.id}"]
min_size = 1
desired_capacity = 1
max_size = 3
launch_configuration = "${aws_launch_configuration.lc.name}"
target_group_arns = ["${aws_lb_target_group.lbtg.arn}"]
default_cooldown= 180
health_check_grace_period = 180
termination_policies = ["ClosestToNextInstanceHour", "NewestInstance"]
}