numpy.core. _exceptions.Ufunctypeerror: ufunc 'add' Did not contain a loop with Signature matching types (dtype('<U38')

Question

i am starting in python and for my first 'big project' I decided to create a program that encrypts, with double key, a message according to the character indexes in an ascii character list. but when I’m trying to add together the matrices that represent each key is giving the following error:

numpy.core. _exceptions.Ufunctypeerror: ufunc 'add' Did not contain a loop with Signature matching types (dtype('<U38'), dtype('<U38')) -> dtype('<U38'')

I tried to add the matrices with '+' too and it didn’t work

import numpy as np
print('-=' * 20)
print('             Encriptador')
print('-=' * 20)

# definindo matriz que vai gerar a chave pública
a = np.random.randint(1, 10, size=(5, 5))

chave_publica = []

alfabeto = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm',
            'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z',
            ' ', '!', '"', '#', '$', '%', '&', "'", '(', ')', '*', '+', ',',
            '-', '.', '/', ':', ';', '<', '=', '>', '?', '@', '[', '/', ']',
            '^', '_', '`', '{', '|', '}', '~',  'á', 'à', 'ã', 'é', 'è', 'í',
            'ì', 'ó', 'ò', 'ú', 'ù', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ']


# definindo a chave pública
for k in range(0, 5):
    k = a[k, k]
    chave_publica.append(k)
chave_publica = np.array(chave_publica)

print(f''
      f'chave pública >>>>> {chave_publica}')

chave_privada = []

print('(frase deve ter mais que 5 caracteres)')

msg = (str(input('frase: ')).strip()).lower()


# a chave privada vai depender do tamanho da mensagem
for k in range(0, len(msg)):
    for j in range(0, len(msg[k])):
        letra = alfabeto.index(msg[k][j])
        chave_privada.append(letra)
chave_privada = '[' + (str(chave_privada).strip('[')).strip(']').replace(',', '') + ']'
chave_privada = np.array(chave_privada)
print(f'chave privada >>>>> {chave_privada}')

l_chave_pub = len(str(chave_publica))
l_chave_priv = len(str(chave_privada))


# definindo a chave para criptografada
if l_chave_priv > l_chave_pub:
    n = l_chave_priv // l_chave_pub
    r = l_chave_priv % l_chave_pub
    n_chave_pub = np.repeat(chave_publica, (n-1)) + ([0] * r)
    c_def = np.add(chave_privada, n_chave_pub)
    print(c_def)
else:
    n = l_chave_pub // l_chave_priv
    r = l_chave_pub % l_chave_priv
    n_chave_priv = np.repeat(chave_privada, n) + ([0] * r)
    c_def = np.add(n_chave_priv, chave_publica)

print(c_def)