def decodeMorse(morse_code):
    morse_W = '.--'
    letter_W = MORSE_CODE[morse_W]
    print(f'letter = {letter_W}')

How to use the preloaded dictionary in python?

Probably. Nested loops grow extremely fast when the input grows, while flat loops just grow proportionnally.
See:

• 3 + 3 + 3 = 9
• 3 * 3 * 3 = 27
• 10 + 10 + 10 = 30
• 10 * 10 * 10 = 1000

etc.

I think your question has received an answer :)

Really? if i will have third but not nested loop, will not it time out?

You can also technically still have O(n) time with mutliple loops, as long as they are NOT nested. This would be O(2n), which is still considered O(n).

Great Kata, but isn't it too hard for 6 kyu? Great kata nevertheless.

i feel f stupid after seeing this.

Roughly saying, yes. If both of nested loops have linearly more iterations if n gets linearly larger, it's indeed O(n^2).

Does O(n) time complexity means having just one loop? while O(n^2) have nested loops?

Too much tests. my code isn't efficient enough.