terraform 初心者です。

結論

terraform apply は main.tf と同じディレクトリではないと、実行できない。

Test1
└── Test2
     ├── main.tf
     └── terraform.tfstate

$ cd Test1
$ ls
terraform_test

# main.tf より上の階層（Test1）で terraform apply を打つと…
$ terraform apply
Error: No configuration files

Apply requires configuration to be present. Applying without a configuration
would mark everything for destruction, which is normally not what is desired.
If you would like to destroy everything, run 'terraform destroy' instead.

うまくいきません。
そこで、階層を変えます。

~ master*
$ cd Test2
$ ls
main.tf           terraform.tfstate

# main.tf と同じ階層（Test2）で terraform apply を打つと…
$ terraform apply   
aws_instance.example: Refreshing state... [id=i-0dde93a30067c72a1]

An execution plan has been generated and is shown below.
Resource actions are indicated with the following symbols:
  ~ update in-place

Terraform will perform the following actions:

  # aws_instance.example will be updated in-place
  ~ resource "aws_instance" "example" {
        id                           = "i-0dde93a30067c72a1"
      ~ tags                         = {
          + "Name" = “Hello”
        }
        # (26 unchanged attributes hidden)




        # (4 unchanged blocks hidden)
    }

Plan: 0 to add, 1 to change, 0 to destroy.

Do you want to perform these actions?
  Terraform will perform the actions described above.
  Only 'yes' will be accepted to approve.

  Enter a value: yes

aws_instance.example: Modifying... [id=i-0dde93a30067c72a1]
aws_instance.example: Modifications complete after 1s [id=i-0dde93a30067c72a1]

Apply complete! Resources: 0 added, 1 changed, 0 destroyed.

無事に実行されました。
ちなみに、上述のコードは、EC2インスタンスのタグをHelloにしようとしています。
お疲れ様でした。