Python - username and password authentication
# importing pickle


Obviously. Don't clutter your code with comments unless they add something we don't already know.

# defining variables
create_username = 0
create_password = 0
password = 0
username = 0


This is generally a bad idea, and you aren't in mid-1990s C. Don't predeclare your variables. Initialize them where they're actually used.

print(users)


If users is a plain-old dictionary, I recommend the use of pprint instead of print.

"enter username(type signup to create an account): "


This is called in-band control, and is a bad idea. What if someone's username is called "signup"? You want out-of-band control. Ask explicitly whether the user wants to sign up or log in.

if username == "signup" or username == "Signup":


Don't do two comparisons. Convert username to lowercase and compare with 'signup'.