terraform interpolation with variables returning error

Question

I am trying to add variables in string

# variables.tf file
terraform {
  backend "s3" {
      bucket    = terraform-20200527"
      workspace_key_prefix  = "customer"
      key   =  "env/${env_name}/terraform.tfstate"
      region = "us-east-1"
      profile   = "default"
  }
}

getting error:

Error: Variables not allowed

   on variables.tf line 7, in terraform:
       7:     key   =  "env/${env_name}/terraform.tfstate"

Variables may not be used here.