workspace.LevelUp.OnServerEvent:connect(function(player) game.Players.player.StarterGui.MainGui.LevelUpFrame.Visible = true end)
1.You can only use OnServerEvent in serverscripts 2.You have to connect the event not 'workspace.LevelUp' 3.You can't call "Players.player" thats gonna be an error
Since in the function(player) you already called a player, you could just use player.StarterGui instead of game.Players.player.
This should be the correct code :
|ServerScript|
local Event = writewhereyoureventislocated Event.OnServerEvent:connect(function(player) player.StarterGui.MainGui.LevelUpFrame.Visible = true end)
Hope this helps!