Okay let’s follow that line of thought.
AuthorityHandlePartyOwnerStateUpdated is triggered by a syncVar hook, so first thing is to check that the hook is setup correctly. Make sure that is spelled correctly and everything.
[SyncVar(hook = nameof(AuthorityHandlePartyOwnerStateUpdated))]
private bool isPartyOwner = false;
Next, make sure that value is being updated correctly to trigger the hook. So in the OnServerAddPlayer method in the RTSNetowrkManager, check that it has the line at the end:
player.SetPartyOwner(Players.Count == 1);
Should have that double equation to make sure player 1 is always the host and has access to the start button.
Let me know if that is all correct.
Side note, does your OnClientConnect() include the base.OnClientConnect()?