I do you not understand

Question

list =[1, 1, 2, 3, 5, 8, 13]
print(list[list[5]])
result : Error

Answer 1

Look at the error: it says IndexError – that means you are trying to index an element at an index that doesn't exist.

Answer 2

print(list[list[5]])

Is equal to

print(list[8]) === null

Answer 3

Indexing list at list[5], probably a duplication of code.

The inner inex should work fine list[5] = 8

Meaning the outer index becomes list[8]

Your array items are as follows:
0 1 2 3 4 5 6
1 1 2 3 5 8 13

So there is no index 8, and calling it results in a type None which is python's null type, null types cannot be printed.