Define variable in while loop

You have, more or less, the right idea. It's just that you need to assign a value to password_length outside your loop.

Think about this: when your code is run, the interpreter hits the while loop and attempts to make a comparison involving password_length. However, at that point password_length doesn't exist yet, since the first time it gets a value is inside the loop. Therefore, you should initialise it to a sensible value, such as 0, before entering the loop.

Two supplementary points:

1. You're calculating the password length of the previous password, so if you enter a too-short/long password and then an acceptable one, the length printed will be of the unacceptable one.

2. In general, prefer f-strings or str.format calls to string concatenation, so, for your print, this might be better:

print(f'Number of characters used in password: {password_length}; '
       'the min length expected is {MIN_PASSWORD_LENGTH} and the '
       'max length expected is {MAX_PASSWORD_LENGTH}.')

Immediately before the while loop, initialize with something like "password_length = MAX_PASSWORD_LENGTH" or else the while loop cannot start. The first line inside the while loop is "password_length = len(password)", which will set the value of password_length correctly, but the while loop needs something to start with so that you can reach that point.

The problem is your calling password before the code knows what it is.

MIN_PASSWORD_LENGTH = 6

MAX_PASSWORD_LENGTH = 14

# password must be defined before you use it. 
password = input("Enter your password: ")

while password_length >= MIN_PASSWORD_LENGTH or password_length <= MAX_PASSWORD_LENGTH:

password_length = len(password)  

if password.isalpha(): 
    print("Your password is weak!")

elif password.isnumeric(): 
    print("Your password is weak!")

else: 
    print("Your password is strong!")   

print("Number of characters used in password: ", password_length,"the min length expected is: ",MIN_PASSWORD_LENGTH,
"the max length is: ", MAX_PASSWORD_LENGTH)

You're using password and password_length variables before they are defined.

Also you can use function and rewrite it more structured:

MIN_PASSWORD_LENGTH = 6
MAX_PASSWORD_LENGTH = 14

def checkPass():
    password = input("Enter your password: ")
    password_length = len(password)

    if password.isalpha():
        print("Your password is weak!")
    elif password.isnumeric():
        print("Your password is weak!")
    else:
        print("Your password is strong!")

    return password_length >= MIN_PASSWORD_LENGTH or password_length <= MAX_PASSWORD_LENGTH

while checkPass():
    continue

print("Number of characters used in password: ", password_length,"the min length expected is: ",MIN_PASSWORD_LENGTH, "the max length is: ", MAX_PASSWORD_LENGTH)

Online demo

Here is a version based on what I understood

MIN_PASSWORD_LENGTH = 6
MAX_PASSWORD_LENGTH = 14
password = ""
atLeast1Apha = False
atLeast1Numeric = False

while True:
    password = input("Enter your password: ")   
    if len(password) > MAX_PASSWORD_LENGTH or len(password) < MIN_PASSWORD_LENGTH:
        print("Password could be between %s and %s long" %(MIN_PASSWORD_LENGTH, MAX_PASSWORD_LENGTH))
        continue
    for char in password:
        if char.isalpha():
            atLeast1Apha = True
        if char.isdigit():    
            atLeast1Numeric = True
    break        

if atLeast1Apha and atLeast1Numeric:
    print("Your password is strong!")
else:
    print("Your password is weak!")

Conditions in the while loop are wrong. You may try:

MIN_PASSWORD_LENGTH = 6

MAX_PASSWORD_LENGTH = 14

password = input("Enter your password: ")
password_length = len(password)

while password_length < MIN_PASSWORD_LENGTH or password_length > MAX_PASSWORD_LENGTH:
    print("Password length", password_length, "incorrect. Required between", MIN_PASSWORD_LENGTH, "and", MAX_PASSWORD_LENGTH)
    password = input("Enter your password again: ")
    password_length = len(password)

if password.isalpha() or password.isnumeric():
    print("Your password is weak!")
else:
    print("Your password is strong!")

print("Number of characters used in password:", password_length,". The min length expected is:",MIN_PASSWORD_LENGTH, ". The max length is: ", MAX_PASSWORD_LENGTH)