Hi@akhtar,
You need to specify "access_config" under "network_interface" to assign external IP to the GCE instance. 
resource "google_compute_network" "default" {
  name = "my-network"
}
resource "google_compute_subnetwork" "default" {
  name          = "my-subnet"
  ip_cidr_range = "10.0.0.0/16"
  region        = "us-central1"
  network       = google_compute_network.default.id
}
resource "google_compute_address" "internal_with_subnet_and_address" {
  name         = "my-internal-address"
  subnetwork   = google_compute_subnetwork.default.id
  address_type = "INTERNAL"
  address      = "10.0.42.42"
  region       = "us-central1"
}