Derivative of a constant involving pi

Even though it involves the symbol $\pi$, the quantity $\pi^2$ is a fixed real number (approximately $9.87$), not a function of $x$. There is no $x$ anywhere in the expression, so $f(x)$ does not change as $x$ changes.

By the constant rule, $\frac{d}{dx}[c] = 0$ for any constant $c$. Applying this with $c = \pi^2$ gives $f'(x) = 0$.