I mean go to your Clan menu and press the "create kingdom" option at the top right and ignore the main quest. I don't think you can get anything extra from the main quest and even later parts have bugs too, so you might as well skip it.
I completed all the quests. I talked to Arzagos and Istiana and completed their quests.
I have more than 100 soldiers.
My clan is independent and level is more than 4.
I have 3 cities and 3 castles and one of them from Imperia.
but when I went to talk to Arzagos or Istiana to build kingdom, I...