word = "Привет"
shifr = "642513"
def mask_data(in_word, in_key):
    result = ""
    wordlist = [x.lower() for x in in_word]
    if len(in_word) == len(in_key):
        for y in in_key:
            try:
                key = int(y)
                if (key - 1) >= len(in_word) or (key - 1) < 0:
                    print(f"The key {key} is wrong")
                else:
                    result += wordlist[key - 1]
            except expression as identifier:
                print("Shifr has error chars")
    else:
        print("The key and word length isnt matched")
    print(f"Rsult: {result}")
mask_data(word, shifr)

  
>>> def encrypt_text(text, key):
...     if len(text) != len(key):
...         raise ValueError('incorrect length of key')
...     out = ''.join(text[i-1] for i in map(int, key))
...     return out
... 
>>> encrypt_text('Привет', '642513')
'твреПи'
>>> 
>>> encrypt_text('Привет', '64251')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in encrypt_text
ValueError: incorrect length of key
>>>