Weird Output for my Map Changer?

That error occurs after you destroy an object game.Workspace.ChosenOne:Destroy() and then try to parent that same object to the workspace.

-If this is the whole script, note that you're only choosing a random map once, which means one server will only ever use one map. Instead, move the lines where you choose the map into the destroy.Changed event, or - if you want the map to activate immediately - into their own function, like this:

local modelGroup = game.ReplicatedStorage:FindFirstChild("Maps"):GetChildren()
local Model
local destroy = game.ServerScriptService.RoundStats.ShouldDestroy
local damage = destroy.Parent.MostDamage
function ActivateNextMap()
	if Model then Model:Destroy() wait(0.3) end --wait is optional, but you had it below
	Model = modelGroup[math.random(1, #modelGroup)]:Clone()
	Model:MakeJoints()
	Model.Parent = workspace
	--If you want, you can set Model.Name here as well, though it's not necessary for this script
end

destroy.Changed:connect(function(property)
	if not destroy.Value then return end
        --[[ Reset Round Stats ]]--
        destroy.Value = false
        damage.Value = ''
        --[[ End Reset Sequence ]]--
	ActivateNextMap()
	----------
	local pl = game.Players:GetChildren()
	for i=1,#pl do
	    pl[i].Character.Humanoid.Health = 0
	end
end)
ActivateNextMap() --thus, 'destroy' doesn't have to become true before the first map is created

Also note that you had some of your code in the player-reset-health loop. Proper indentation shows where everything is.