How to make a server lock command and restrict only certain players to use it and be immune to it?

So you want certain players to run the command, and also want certain players to be immune. There is multiple ways to make certain players immune, I assume you have a group, so I'll use player:GetRankInGroup. You can use Player.UserId (or even Player.Name, however I do not suggest using this one.) Another thing I should say is that you should preset the BoolValue to false using the properties tab instead of setting the value via script.

local SS = game.ServerStorage

game.Players.PlayerAdded:Connect(function(plr)
	local slocked = SS.Slocked
	if slocked.Value == true then --Place it at the top and NOT under the Chatted event.
		if not plr:GetRankInGroup(0) == 255 then --If it's not the owner
			plr:Kick("The training center is closed! Please attend it at the scheduled time.")
		end
	end
	plr.Chatted:Connect(function(msg)
		if msg == "lockserver" and slocked.Value == false then
			if plr:GetRankInGroup(0) == 255 then --This is the owner rank. Replace the 0 with the groupID or just use the alternative mentioned.
				slocked.Value = true
			end
		end
		if msg == "unlockedserver" then
			if plr:GetRankInGroup(0) == 255 then
				slocked.Value = false
			end
		end
	end)
end)