检查实例是否*不存在*->"非法成员"

"checkin不是PlayerGui的一个有效成员"，位于第2行

function onClick(plr)
if game.Players[plr.Name].PlayerGui.checkin ~= nil then
print('player already has gui')
else
if game.ServerStorage.Players[plr.Name].Value == 0 then
local gui = game.ServerStorage.GUIs.checkin:Clone()
gui.Parent = plr.PlayerGui
print('fresh gui being handed to '.. plr.Name)
end
end
end
script.Parent.ClickDetector.MouseClick:connect(onClick)