Hi@akhtar,
You need to use aws_subnet resource inside your VPC. This will allow you to create a subnet inside your VPC. I have attached one example below for your reference.
resource "aws_subnet" "main" {
vpc_id = "${aws_vpc.main.id}"
cidr_block = "10.0.1.0/24"
tags = {
Name = "Main"
}
}