Hi@akhtar,
You can create a folder at the time of creating an S3 bucket. You will find one resource aws_s3_bucket_object that will help us to create one folder inside the S3 bucket. I have attached one example below for your reference.
resource "aws_s3_bucket_object" "folder1" {
bucket = "${aws_s3_bucket.b.id}"
acl = "private"
key = "Folder1/"
source = "/dev/null"
}