What does your code do for this?

From the description:
"The path ["NORTH", "WEST", "SOUTH", "EAST"] is not reducible."
For that case, the input and output should be the same.

Read the description more closely.

First example:

E W N S N N N E W S N
E W                   == 0
    N S               == 0
              E W     == 0
                  S N == 0
        N N N 

By eleminating E W and S N combinations N N N remains. Nothing wrong with this kata.

You could, you know, read the description carefully, for example, or something. But yeah, complaining is also an option ;)

Post your code with proper markdown and a spoiler tag and I'll take a look.

I have also this problem, I am coding Java.

Which programming language? Which tests?
What is the code you used?

I believe it is because you have not split the string into separate letters with str.split(''). Therefore when you call str[i] without it being an array you get an error message.