Droplets created by Terraform cannot ping each other

Terraform

resource "digitalocean_droplet" "api" {
  image              = "ubuntu-18-04-x64"
  name               = "api"
  region             = "SGP1"
  size               = "s-2vcpu-2gb"
  count              = 2
  private_networking = true
  tags               = ["api", "www"]
  ssh_keys           = module.root_ssh.root_ssh_ids
}

When I try root@api_1$ ping api_2 there’s no packet returned. I had cloud firewall but not enable it for these droplets and ufw status result inactive