double dval = 1;
for (int i = 0; i < Cols; i++)
k = 0;
dval = 1;
for (int j = Cols - 1; j >= 0; j--)
colIndex = (i + j) % 3;
val *= dval[colIndex, k];
det -= dval;
Cannot apply indexing with  to an expression of type 'double' for dval
dval is not an array, so you cannot use it in that way. you should have
double [,] dval = new double[rows, cols];
somewhere in your code, to define the matrix you are trying to use, and then you must have put some data in it.
Which kind of calculation are you trying to do?