This is a completely obvious target.
However Ukraine had no overt weapon with the range and accuracy to hit it, that could get through Russian air defenses.
And having covert operatives in Russia does not mean that its easy to obtain a truckload of explosives from Russian sources, or persuade or subvert the Russians required to make this possible.
You are a thick one! Have you ever heard the term “Wag the Dog”? Its playing out right in front of us.