This error occurs when you want to compare to cells, which you can't do. What you probably want is to compare the contents of the cells. For this put the corresponding index after the cell:
X = {'a', 'b'};
Y = {'a', 'c'};
X{1} == Y{1} % compare 'a' to 'a' => works
X{2} == Y{2} % compare 'b' to 'c' => works
X == Y % this throws the error