winner = board[0][i];
winner
is an int primitive type and board
is multi-dimentional String Array.
you are trying to assign a string for an int thus incompatible error for Int and String.
String[][] board;
has Strings at its indexes, when you try to access board[0][i]
you are retrieving a String.
if your board array contains String representation of numbers like
boards= {{"1"},{"2"}};
then use Integer.parseInt() which takes String as an argument and returns an Integer.
winner = Integer.parseInt(board[0][i]);
but mind you that if the String passed to parseInt is not a valid integer represetation of a string it'd throw NumberFormatException.