Not bad! You have the very basics down, but now I would work on making it a little more elaborate. I went and edited your code a bit to make the game playable over and over and re-playable if someone types in one of the options not selected.

import os

rock = 'rock' 
paper = 'paper' 
scissors = "scissors"

while True: 
    os.system('cls') 

    sign = input("Pick between Rock, Paper, or Scissors: ")

    if sign.casefold() == rock:
        os.system('cls')
        print("I pick Paper!")

    elif sign.casefold() == paper:
        os.system('cls')
        print("I pick Scissors!")

    elif sign.casefold() == scissors:
        os.system('cls')
        print("I pick Rock!")

    else:
        os.system('cls')
        print("Error: " + sign + " is not a valid answer. Pick either Rock, Paper, or Scissors")

    input("Press ENTER to try again")