Update for subnets.tf, instances.tf and security_groups.tf:

In Terraform versions below the 12, the following syntax was used for “tags”:

tags {
   Name = "MY_TAG_NAME"
}

Currently, you need to add “=” after “tags”:

tags = {
   Name = "MY_TAG_NAME"
}

Line #2 of instances.tf should be:

subnets = flatten(aws_subnet.terraform-blue-green.*.id)

instead of:

subnets = ["${aws_subnet.terraform-blue-green.*.id}"]