The code write like this cannot work, the for loop need spaces. Something like this:
letter = ['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',' ']
number = [8,26,13,4,4,3,26,0,26,5,14,17,4,8,6,13,26,5,17,8,4,13,3]
message = ""
for i in number:
message += letter[i]
Another thing you need to print the output otherwise it will not print anything:
letter = ['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', ' ']
number = [8, 26, 13, 4, 4, 3, 26, 0, 26, 5, 14, 17, 4, 8, 6, 13, 26, 5, 17, 8, 4, 13, 3]
message = ""
for i in number:
message += letter[i]
print(message)
One last thing, please read the “Conventions for the Python code” here: https://www.python.org/dev/peps/pep-0008/
The code will be more readable and beauty if you follow it:
letter = ['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', ' ']
number = [8, 26, 13, 4, 4, 3, 26, 0, 26, 5, 14,
17, 4, 8, 6, 13, 26, 5, 17, 8, 4, 13, 3]
message = ""
for i in number:
message += letter[i]
print(message)
If you do that we can be friends 
-Gabriele-