In reply to jyothsna04:
The code you wrote and the code in the marked solution is illegal. The reduction methods only iterate over a single dimension of an array
constraint c2 {
foreach (a[i]) {
a[i].sum() with (int'(item)) == 10; // rows a[i][0] + a[i][1]
a.sum() with (int'(a[item.index][i]))== 10; // cols a[0][i] + a[1][i]
}
// unique{a}; // array size and sum too small to make unique
}