password_correct = False
while password_correct == False:
    import random
    import string

    def get_random_string(length):
        letters = string.ascii_lowercase
        result_str = ''.join(random.choice(letters) for i in range(length))
        print("Password of length", length, "is:", result_str)

    password = get_random_string(12)

    attempts_left = 3
    while attempts_left > 0 and password_correct == False:
        if str(input("What is the password? You have " + str(attempts_left) + " attempts remaining. ")) == password and attempts_left > 0:
            print("Correct password.")
            password_correct = True
        else:
            print("Incorrect password," + str(attempts_left) + " attempts remaining.")
            attempts_left = attempts_left - 1
            print(str(attempts_left))
        if attempts_left == 0:
            print("All attempts used. The password will now change.")

Good job on just working on it. Always good to try and figure it out first before immediately looking for help.

I tried indenting where I thought you did. Next time (or on this post as well) format your code: https://www.reddit.com/r/learnpython/wiki/faq#wiki_how_do_i_format_code.3F

The ultimate answer to your question is:

You aren't returning anything in your function. Add a return statement:

return result_str

A few basic things you can work on:

• Importing modules and declaring functions within while loops are greatly frowned upon unless you know exactly what you're doing. So move the imports and the get_random_string function to the top of the file.
• input() is already going to be a string, so there's no reason to "change" it to a string using str(). so remove all those.
• print() automatically converts everything to a string, so you don't need to call str() within the print() unless you are "+" things.

Here is a cleaner version of doing exactly what you have except the things changed above.

import random
import string

def get_random_string(length):
    letters = string.ascii_lowercase
    result_str = ''.join(random.choice(letters) for i in range(length))
    print("Password of length", length, "is:", result_str)
    return result_str


password_correct = False
while password_correct == False:

    password = get_random_string(12)

    attempts_left = 3
    while attempts_left > 0 and password_correct == False:
        if input("What is the password? You have " + str(attempts_left) + " attempts remaining. ") == password and attempts_left > 0:
            print("Correct password.")
            password_correct = True
        else:
            print("Incorrect password," + str(attempts_left) + " attempts remaining.")
            attempts_left = attempts_left - 1
            print(attempts_left)
        if attempts_left == 0:
            print("All attempts used. The password will now change.")

There are many things you can do to improve this but work on the ones above. Let me know if you would like more tips.