[2023] GAME IS NOT IN SYNC

Are there any mods in use?

When you hit PLAY in the Steam Library, you get a mini-menu. In there choose Nakama instead of Steam MP - all players will need to do this.
Does that help matters?  

Have all players check that all the Steam ports are open (NOT forwarded, open) in all their firewalls:

Steam Support :: Required Ports for Steam

support.steampowered.com
Good luck!