User data terraform

maybe try this way:

resource "aws_instance" "example" {
  # other instance configuration...

  user_data = base64encode(local.user_data_script)
}

locals {
  user_data_script = <<EOF
#!/bin/bash
apt update -y
apt install -y apache2
systemctl enable apache2
systemctl start apache2
echo "<h1>Welcome to Structure AWS with Terraform </h1>" > /var/www/html/index.html
EOF
}

Place your UserData into a Shell script file and then reference it instead of placing it in the Raw terraform:-

/script.sh

#!/bin/bash
apt update -y
apt install -y apache2
systemctl enable apache2
systemctl start apache2
echo "<h1>Welcome to Structure AWS with Terraform </h1>" | tee /var/www/html/index.html

resource "aws_instance" "web" {
  ami                    = var.image_id
  instance_type          = var.instance_type
  key_name               = aws_key_pair.key-tf.key_name
  vpc_security_group_ids = ["${aws_security_group.allow_tls.id}"]
  tags = {
    Name = "first-tf-instance"
  }
  user_data = file("${path.module}/script.sh")
}