Answer
The $Ba(OH)_2$ concentration should not be in the $K_{sp}$ expression, and the ion concentrations should be in the numerator of the fraction (top).
Correct version:
$K_{sp} = [Ba^{2+}][OH^-]^2$
Work Step by Step
When we write the $K_{sp}$ expression, we do not consider the concentration of the solid salt. And, the $K_{sp}$ expression is the fraction of: $\frac{[Products]}{[Reactants]}$.
