Python prematurly shuts down.

I have tried your code in Python 3.6, and in the IDLE IDE, and in both cases it correctly shows an error message. Firstly you have an extra indent level after the first line. Once you correct that then you see that you are using the variable name username without previously declaring it.

The following code appears to work. Note I have commented out the parts that write the details to the file, and most of the sleep calls (why are they there?).

import time

def password():
    ans = "no"
    while ans == 'no' or ans == 'n':
        a = 0
        password = input("Enter a password. \n(Must contain at least 1 number, 1 captial, 5 characters and have no special characters)\n")
        if password.isalnum() == True:
            if password != password.lower():
                if len(password) > 4:
                    for i in range(len(password)):
                        try:
                            float(password[i])
                        except:
                            a = a
                        else:
                            a = a + 1
                    if a > 0:
                        ans = input("Your password is " + password + "\nIs this the password you wanted? Y/N\n")
                        ans = ans.lower()
                        if ans == 'y' or ans == 'yes':
                        #    f = open("Password.txt", "w")
                        #    f.write(password)
                            print("Your password is being added to the database.\n")
                        #    time.sleep(1)
                            print("...\n")
                        #    time.sleep(1)
                            print("Your password has been set.")
                        #    time.sleep(2)
                        #    f.close()
                        else:
                            print("Test failed!")
                    else:
                        print("You need at least 1 number.")
                        #time.sleep(2)
                else:
                    print("You must have at least 5 characters.")
            else:
                print("You need at least 1 capital.")
#                time.sleep(2)
        else:
            print("That contains a special character.")
#            time.sleep(2)
 
def username():
    ans = "no"
    while ans == "no" or ans == "n":
        username = input("What do you want your username to be?\n")
        if username.isalnum() == True:
            if len(username) > 4:
                ans = input("Your username is " + username + " is this correct? Y/N\n")
                ans = ans.lower()
                if ans == "y" or ans == "yes":
 #                   f = open("Username.txt", "w")
 #                   f.write(username)
                    print("Your username is being added to the database.\n")
 #                   time.sleep(1)
                    print("...\n")
 #                   time.sleep(1)
                    print("Your username has been set.")
 #                   time.sleep(2)
 #                   f.close()
                    password()
                else:
                    print("Test failed!")
            else:
                print("Your username has to have more than 4 characters.")
 #               time.sleep(2)
        else:
            print("You username cannot have a special character.")
 #           time.sleep(2)

username()