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"
  }
}