The issue with your code is that you put the comma with the "paper" string instead of putting it after the string. Because there is no comma between paper and scissors, the computer interprets them as one element, meaning the list has a length of 2, not 3. This is the full code. I hope it helps:
from random import choice
# getting input from the user
user_choice = input("Rock, paper or scissors? ").lower()
# printing an error message and quiting the program if the input is invalid
if user_choice != "rock" and user_choice != "paper" and user_choice != "scissors":
print("Error! Invalid answer!")
quit()
# generating the computer's choice
options = ["rock", "paper", "scissors"]
computer_choice = choice(options)
print("The computer chose", computer_choice)
# checking all of the possible options and declaring who the winner is
if user_choice == computer_choice:
print("You both chose", computer_choice, "therefore it's a tie!")
elif user_choice == "rock" and computer_choice == "paper":
print("Paper beats rock, therefore the computer won!")
elif user_choice == "paper" and computer_choice == "rock":
print("Paper beats rock, therefore you won!")
elif user_choice == "rock" and computer_choice == "scissors":
print("Rock beats scissors, therefore you won!")
elif user_choice == "scissors" and computer_choice == "rock":
print("Rock beats scissors, therefore the computer won!")
elif user_choice == "paper" and computer_choice == "scissors":
print("Scissors beats paper, therefore the computer won!")
elif user_choice == "scissors" and computer_choice == "paper":
print("Scissors beats paper, therefore you won!")