root module does not declare a variable of that name. To use this value, add a "variable" block to the configuration

my directory structure

.├── README.md├── ec2│  ├── ec2.tf│  ├── outputs.tf│  └── vars.tf├── main.tf

main.tf

provider "aws" {  region = "us-east-1"}module "ec2" {  source = "./ec2"}

ec2/ec2.tf

data "aws_ami""example" {  most_recent = true  owners = ["amazon"]  filter {    name = "image-id"    values = ["ami-0323c3dd2da7fb37d"]  }  filter {    name = "root-device-type"    values = ["ebs"]  }  filter {    name = "virtualization-type"    values = ["hvm"]  }}resource "aws_instance""web" {  ami = data.aws_ami.example.id  instance_type = "t2.micro"  subnet_id = var.subnet_id  tags = {    Name = "HelloWorld"  }}

ec2/avrs.tf

variable "subnet_id" {  default = {}}

when i try to pass the subnet_id from outside i'm getting error.

terraform plan -var subnet_id=$subnet_name

Error: Value for undeclared variableA variable named "subnet_id" was assigned on the command line, but the rootmodule does not declare a variable of that name. To use this value, add a"variable" block to the configuration.

if any of you have idea about this issue please help me.