Speed Typing Test in Python errors

This is my version of speed typing test in Python. I am getting the following errors and I can't seem to understand them.

Shadows name 'random_sentence' from outer scope
Local variable 'user_input' value is not used
Shadows name 'user_input' from outer scope
Local variable 'i' value is not used

This is my python script

from sentence_list import sentences
import random
import time

global user_input, random_sentence


def time_block():
    random_sentence = random.choice(sentences)
    print(random_sentence)
    print("\n")
    start_time = time.time()
    user_input = input()
    end_time = time.time()
    time_difference = end_time - start_time
    print("Total time taken: ", round(time_difference, 2))


def error_block():
    error_count = 0
    for i in user_input.split(" "):
        if i not in random_sentence.split(" "):
            error_count += 1
    print("Total number of errors made: ", error_count)


def words_per_minute():
    word_count = 0
    for i in user_input.split(""):
        word_count += 1
    wpm = word_count / 5
    print("Words Per Minute (WPM): ", wpm)


time_block()
error_block()
words_per_minute()

I can't post the sentences file because it is too big. Please be free to use your own sentences and help me sort out this problem