Encrypting program

Question

Hi,
I have a problem with my new code. How can I upgrade it that it will accept numbers and signs like ".".
Main purpose of this code is to encrypt messages lets say like enigma :) It should give as output only alphabetical characters.

Feel free to check this out:
https://code.sololearn.com/clRm5DuRQsMG/#py

I am new here and new at codding so I will be happy to get feedback :)