I just need help fixing my code(Python using replit) so the login won’t keep having an index error and that the user can login or be told to try again

I used a sub program for the login this is the code I did:

def login(login):
 access = False
 while access == False:
  print ("**Login Loading**\\n")
  username = input("Enter your username: \\n")
  password = input ("Enter your password: \\n")
  file = open("Valid_users.txt","r")
  valid_users = file.read()
  valid_users = username.replace('\\n', ' ').split(".")
  for line in open("Valid_users.txt", "r").readlines():
#usersfile = line.split()
   if username == valid_users\[0\] and password ==  valid_users\[1\]:
    print("Welcome!")
   else:
    print("Username or password incorrect. Try again")
 if access == True:
  print("logged in")
return username
return access
return login
login(login)

The problem is when I try to search the valid users in the file it come with an index error I’m not very familiar with sub programs so I’m not sure what to do.

This is my registration one that adds to the list:

def register():
 print("***Registration Loading***\\n")
 username = input("Enter Username\\n")
 while len(username)\<4 or len (username)\>20:
  print("Username does not meet length criteria.")
  username = input("Enter Username\\n")
 password = input ("Enter Password\\n")
 while len(password) \<5 or len (password) \>20:
  print("Password does not meet length criteria")
  password = input ("Enter Password\\n")
 file = open ("Valid_users.txt","a+")
 file.write(username+","+password+"\\n")
 print("Registration Completed")
 file.close()
register()

I tried changing the range and variables a few times I also tried taking gaps out of the lines of where the username and password is and that still didn’t work

source https://stackoverflow.com/questions/76689805/i-just-need-help-fixing-my-codepython-using-replit-so-the-login-won-t-keep-hav