Few thought the Russians would actually do it.
And the Ukrainians truly weren’t ready. On the Belorussian border there was nothing but border guards and militia - which hadn’t been mobilized. There was only the usual peacetime station active army brigade near Kiev, and another at Chernighiv, and just a platoon at Sumy. At the defiles out of Crimea there was also just the one brigade, and this was way out of position.
Add to that the whole Ukrainian Air Defense system was in its peacetime state, in storage. Thats why for the first few days the RuAF was able to overfly Ukraine at will.
The NATO training and the very few (at the time) NATO weapons became significant only after the initial clashes had delayed and blunted the Russian advance. That all really began to matter in the next few weeks.
But the Russians SHOULD have won in the first three days.
“Few thought the Russians would actually do it.”
Such nonsense.