How to manage string index out of range in Python?

Your code is unreadable: when you paste code, you need to use the "Paste as" option "Code Block", or use the codewidget on the toolbar above the text box. That tells the system that < and > characters are part of your code and shouldn't be interpreted as HTML:
While(i<L)will be truncated at the < character as it encloses HTML tags in browsers - as can be seen in yoru code.
Enclosed, it looks like this:

n=int(input())
L=len(n)
i=0
while(i<L)
   print("ABCD"[i])
...

Which is a lot more readable!

To fix your problem, we have to guess what you wrote, but it's probably a simple beginner mistake:
If you have a string of 4 characters, then it's len is 4
But the valid indexes start from zero:

0, 1, 2, 3

So if your loop runs from to to len inclusive you are trying to access a character that doesn't exist, and you get an error.
Reduce the limit by one to L - 1 and the error will go away.


But ... why are you converting the input to a integer, then trying to get the length of it anyway?