bastion subnet_id as string not list

main.tf

@@ -97,5 +97,5 @@ module "bastion" {
 
   vpc_id = "${var.vpc_id}"
   allowed_network="10.0.0.0/16"
-  subnet_public="${module.net.public_subnets}"
+  subnet_id="${element(module.net.private_subnets, 0)}"
 }