Configure an access_logs block with the enabled attribute set to true.

Example Configuration

# aws_elb example
resource "aws_elb" "tr_test_elb" {
  name               = "terraform-test-elb"
  availability_zones = ["us-east-1a"]

  access_logs {
    bucket        = "${aws_s3_bucket.elb_test_bucket.id}"
    interval      = 60
    enabled       = true
  }

  listener {
    instance_port     = 8000
    instance_protocol = "http"
    lb_port           = 80
    lb_protocol       = "http"
  }

  health_check {
    healthy_threshold   = 2
    unhealthy_threshold = 2
    timeout             = 3
    target              = "HTTP:8000/"
    interval            = 30
  }

  cross_zone_load_balancing   = true
  idle_timeout                = 400
  connection_draining         = true
  connection_draining_timeout = 400

  tags = {
    Name = "foobar-terraform-elb"
  }
}

# aws_lb example
resource "aws_lb" "tr_test_lb_network_1" {
  name = "lb"
  internal = true
  load_balancer_type = "network"
  subnets = ["${aws_subnet.subnet1.id}", "${aws_subnet.subnet2.id}"]

  access_logs {
    bucket        = "${aws_s3_bucket.elb_test_bucket.id}"
    enabled       = true
  }
}