Structure suggestion tablet puzzles with only one solution

import random

def removeInvalidLetters(nextLetter, code, validLetters):
    '''
    Inputs:
    - The next letter in the code that already appeared in the code
    - The code
    - The current list of valid characters
    Output:
    - An updated list with valid characters. The nextLetter and all characters after it are removed
    '''

    indexLetter = code.find(nextLetter)
    for index in range(indexLetter, len(code)):
        character = code[index]
        if character in validLetters:
            validLetters.remove(code[index])

    return validLetters

def generateCode(length):
    '''
    Input: the length of the code you want
    Output: a code of the specified length that has only one solution
    '''

    code = ""
    validLetters = list(map(chr, range(65, 91)))

    for i in range(length):
        nextLetter = validLetters[random.randint(0, len(validLetters)-1)]
        if nextLetter in code:
            validLetters = removeInvalidLetters(nextLetter, code, validLetters)
        code += nextLetter

    return code

print(generateCode(27))