So when i try to show a gui like this . in a script in the ServerScriptService
while true do
wait(10)
for i,v in pairs(game.Players:GetPlayers()) do
v.PlayerGui.GUI.Block.Visible = true
end
end
and after this . it show me this in the output !
17:21:51.288 - GUI is not a valid member of PlayerGui
and by the way the gui is in the playerGui !

2 comments
PlayerGui's descendants are not accessible by server scripts, you have to use a local script to access it
Just a side note, it seems you're making this an infinite loop, but after it becomes visible you don't exactly need to make it visible again every 10 seconds unless something else is making it invisible repeatedly...