Ok, so a few things here...

First I'll give you exactly what you were looking for and explain what I used and some of the changes that needed to be made to your original code. Then I'll explain some inherent issues what what you're trying to do and suggest some areas to read up on/some ways you might want to improve what you've got.

Here's the code you're looking for (while retaining the same flow as what you submitted put above):

Copyimport random

result = ''
choice = ''
message = ''

characters_in_order = [chr(x) for x in range(32,127)]

while choice != 0:
    choice = input("\n Do you want to encrypt or decrypt the message?\n 1 to encrypt, 2 to decrypt or 0 to exit program. ")

    if str(choice) == '1':
        message = input('\nEnter message for encryption: ')
        
        r_seed = input('Enter an integer to use as a seed: ')
        random.seed(r_seed)
        shuffled_list = [chr(x) for x in range(32,127)]
        random.shuffle(shuffled_list)

        for i in range(0, len(message)):
            result += shuffled_list[characters_in_order.index(message[i])]

        print(result + '\n\n')
        result = ''

    elif str(choice) == '2':
        message = input('\nEnter message to decrypt: ')

        r_seed = input('Enter an integer to use as a seed (should be the same one used to encrypt): ')
        random.seed(r_seed)
        shuffled_list = [chr(x) for x in range(32,127)]
        random.shuffle(shuffled_list)

        for i in range(0, len(message)):
            result += characters_in_order[shuffled_list.index(message[i])]

        print(result + '\n\n')
        result = ''

    elif str(choice) != '0':
        print('You have entered an invalid input, please try again. \n\n')

You'll notice that I set a global 'characters in order' list, which is just every ASCII character (32-126) in order. I also imported the 'random' module and used this to shuffle the characters in order according to a seed that the user inputs. As long as this seed is the same on the encryption and decryption end, it will produce the same shuffled list and it should work to encrypt or decipher the same string. Also notice the str() around your input choices. Without that, the user had to input '1', rather than 1 to submit a choice without an error.

All of that said...

1. Notice that the way the new function works is by looking at a character's index in one list and pulling out the character at that index in another. The method you were using, of incrementing or decrementing a character's ASCII code is basic (though not much more basic than this), but it also has a pretty critical flaw, which is that characters on one end or another of the ASCII set wouldn't return ASCII characters. If you were encrypting it at a bit-level, which would be preferred, this wouldn't matter/would be irrelevant, but here you're not going to get the kind of string back that you want if you were to, for example, enter a [space] (ASCII 32) into your plaintext to be encrypted.
2. If you're interested, you might want to read up on symmetric key encryption/DES for some ideas on how encryption is really done, though props on the start/interest and this can certainly be a fun way to create some sort of cryptogram puzzle or something along those lines. I won't pretend to be any kind of expert, but I can at least point you in the write direction. (https://en.wikipedia.org/wiki/Data_Encryption_Standard https://en.wikipedia.org/wiki/Symmetric-key_algorithm)
3. Consider having your code read in a .txt file and print out to a .txt file, rather than using user input for the message.

Again, I'm not an expert by any means and there are definitely some fun uses of the kind of program you're aiming for, just trying to point you in the right direction if this is something that you're interested in. Hope all of that is helpful!