yes, very good point. but it only effects it if the comma is in the last person in the array.

the [::-1] mirror list.

eg:
'Bart, Lisa, Maggie, Homer & Marge' to
'egraM ,remoH ,eiggaM ,asiL ,traB'

then the .replace specify only to change one ',' to '&' one time.
'egraM & remoH ,eiggaM ,asiL ,traB'

then the last [::-1] mirror the whole list again.
'Bart, Lisa, Maggie, Homer & Marge'

str_final+=MORSE_CODE[j] throws error because j == "" is not in dictionary. Where does j come from? for j in i. Where does i come from? for i in letters. How does letters array look when you call this function on " . "? letters == [["", ".", ""]].

I have tried so hard and just couldn't identify where did I try to look for "". Would you please give me some hint?

Not an issue.

You're trying to find "" in MORSE_CODE while there's no such key in it.