Well,
for c in Msg: r= (A*(ord(c)-65)+ B)%26 print(lettre(r), end= " | " )evaluates for 'r' in this numbers
[74, 72, 73, 71, 88]
. When you substract 65 from each of them you get numbers between 9 and 23. The printable characters start from 32 which itself is space. See http://www.asciitable.com/ . These are not letters.