I found Iperios and it is a modded server where you can make your own nation. You get to decide your government style and set laws and ranks. If you do not want to make one you can join a nation that already exists.
It runs on All the Mods 10 ATM10 version 6.1. You have to get whitelisted through their Discord first. After you get in a staff member helps you and gives you a beginner kit so you can start playing.
I found a modded server that runs on All the Mods 10 ATM10 version 5.5 and the whole vibe is nations. You can make your own nation and pick how it is run with your own government laws and ranks.
If you do not want to start from zero you can join a nation that already exists. To get in you join the Discord and ask to be whitelisted then staff helps you get started and gives you a beginner kit.
Make sure you have the ATM10 modpack installed on version 5.5 before you try to play.