I am king since start of server, now I want to be duke of friendly kingdom. How I can delete my kingdom?
go embassy and click abdicate, if you can't do it, you should leave from the beginner's protection.
yeah you need to have 200 pop and deactivate any active treasuries to abdicate
but also it depends if you formed a Union with another kingdom, if you have merged with another King (i.e. 2 kings and 4 dukes in your kingdom) then you cannot disband your kingdom, or leave your king position