void setup() {

  Serial.begin(9600);

  // Seed our random number generator
  randomSeed(analogRead(A0));
}

void loop() {

  String str;
  int num;

  // Ask the user to enter the number of sides on the die
  Serial.println("Enter the number of sides on the die");
  while ( str.equals("") ) {
    str = Serial.readString();
  }

  // Convert input string into a number
  num = str.toInt();

  // Only roll the die if we have a positive number
  if ( num > 0 ) {
    num = random(1, num + 1);
    Serial.print("You rolled: ");
    Serial.println(num);
  }
}

Simulator: https://tinkercad.com/things/0zACluQ1s2r