I completely understand that @MMBaranova, all I can say as a conflict scholar is that sometimes countries do start wars that are very poorly planned and executed.
We can look at the first Russian invasion of Chechnya in 1994, when they thought they would take Grozny in 2 days. But the battle plan was poorly designed, they did not anticipate the stiff Chechen resistance, and in the following weeks and months it was clear that poorly trained and equipped Russian forces were not going to prevail (Russia lost that war).
They went back in 1999 and this time had a better plan for taking Grozny, but had the same problems once again with poorly trained and equipped troops with low morale. They only won that war thanks to Chechen proxies.
The 2008 Russo-Georgian war, Russia essentially won that conflict but it's own internal reviews were scathing about their performance.
(We can also look at Western examples, such as the complete failure of the US/UK to plan for post-invasion Iraqi insurgency.)
Of course we would assume today's invasion would have been carefully planned and prepared for, but it's entirely possible it was not. The forces sent to Ukraine/Belarus were ostensibly sent for short-term exercises. It would have been a very small circle around Putin that planned for invasion instead, and it's entirely possible they are not that competent, or that Putin overrode their advice (as Hitler and Stalin both did).
Military analysts have been saying for a long time that Russia's military is much improved but still has serious flaws, and it's been a very long time since Russia engaged in a large-scale ground campaign. It's not unthinkable that they're making some major mistakes.