What does this mean?

"E" if an error of any kind occured during the interpretation (invalid input or invalid move required)

How do we know who made an error? If someone does not have a legal move, can they make an "invalid" illegal move? Or is the turn flipped and the other made the error? And what if both players don't have a next legal move, and there still is a move in the input, who made the error?

well, it depends how you interpret the description I guess. The description should either:

• state the exact rules, with examples
• OR tell us that it's a prerequisite that we known the rules, and link us to the proper rules somewhere online

I watched a quick guide of the rules. For instance, the statement below is incorrect. You can't flip any pieces in between, you can only flip from the piece you place to your first own piece.

A player can play only if he has a valid move. Valid moves are those in which an opposing piece is trapped (horizontally, vertically, or diagonally) between one already on the board, and the piece being placed. Any pieces between will be flipped (could be more than one!).

For instance

BWWBWW

-> if you place a B at the right, you end up with

BWWBBBB

I think it does, else I would not have given you these specific instructions. But it's up to you to keep it or change it.

I don't know if it's just the description, or also test case generation that is incorrect here

Test cases are unacceptable for a 3 kyu: this solution should not be able to pass

I can't imagine how bad the description was before. It's still a bad description. You don't explain how we should interpret the input. You don't explain how a valid moves looks like.

you have TWO semicolons, please limit it to ONE.

then my solution in JS is wrong, and I was able to pass tests

we might still manage to salvage this kata raul ;)

what is your solution for n = 324?