Putin likely believed the Ukrainians would put up only token resistance. STAVKA apparently planned accordingly, which was pretty fucking stupid of them. You plan for a variety of contingencies. "No plan survives contact with the enemy."
Now STAVKA is reworking their plans, but making fixes won't be easy. In the end, the Russians can win using brute force, at a terrible cost. Reminds me some of my namesake's 1939 attack on Finland. Finns kicked Soviet ass. But in March 1940, with new plans and additional forces, the Soviets were breaking through. Finland sued for peace and got it, at the cost of territory. Made my namesake and the USSR pariahs throughout much of the world. Then the next year the Finns joined the Germans in the attack on the USSR. Basically, the Finns took back everything they lost the year before.