Select single letters from string based on separate string

[ibreeden]

You are asking complicated things. Are you inventing a new encryption mechanism?
You may solve it in this way:

s1 = '085236752048'
s2 = '@ABCDEFGHI'
print(''.join(s2[int(ch)] for ch in s1[6:11]))

Output:
GEB@D

Explanation:
for ch in s1[6:11]: Walk through the numbers in string 1 (mind you: index starts at 0 and last index is not included)
s2[int(ch)]: make an integer of the character found in the loop and consider this as an index in string 2.
''.join(...): start with an empty string and join this successively with the characters found.