random.randrange

[snippsat]

Your nearly there in your code post #3
Just add alphabet[index]

import random

alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmopqrstuvwxyz"
random_characters = len(alphabet)
index = random.randrange(0, random_characters + 1)
print(index)
print(alphabet[index])

A little fancier output

import random

alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmopqrstuvwxyz"
random_characters = len(alphabet)
index = random.randrange(0, random_characters + 1)
print(f'Random characters at index <{index}> give output <{alphabet[index]}>')

Output:
Random characters at index <3> has result of <D>
Random characters at index <1> give output <B>
Random characters at index <24> give output <Y>