I tried to usurp all titles in my realm, but without vassals I cannot form it either. After granting 1 or 2 titles(so I have powerful vassals) I still cannot form it, as game thinks that I do not have hooks or 100 opinion(in addition to illusive "reason")

Think about how difficult it would be to seize the property of feudal vassals with personal armies and castles in an entire realm, framed as a “change of government” system. I don’t think that really happened in this era, and feudalism was slowly eaten away by military and economic paradigm shifts that made the commanding position a feudal lord held less and less important (and less immovable too).

When I want to form an admin government I typically grab Bureaucratic since that lets you do this at very small scales. I haven’t had problems first forming a local kingdom and then going admin. I just hook all the vassals I can and then revoke out the rest. You can also see what unlanded guys you may have strong hooks on and bring them in as vassals.

Hmm may be something wrong with my game then(I'm tried it at kingdom level as Han). Because even without vassals i couldn't form it...

Without vassals at all doesn’t work, I think. My guess is the requirements are positively required, and having no vassals does not satisfy the condition for having +100 with every vassal, a hook on every vassal etc. I’d recommend loading a save from your Han game with debug on, make sure you have 5 powerful vassals, none with ambitious trait, give yourself strong hooks on all of them, make them all +100 opinion, and hopefully then it will work. It may also count your court chaplain so make sure he meets the same conditions.

By doing this you will at least confirm what does work, although obviously without debug those conditions can be very hard to meet. I had a game once where it was not feasible to achieve those conditions so I swore fealty to the ERE to change government that route, then had an independence war a generation later.