game.ReplicatedStorage.CallWarn.OnServerEvent:Connect(function(a,p)
local s = game.ServerStorage:WaitForChild("warning")
local r = s:Clone()
r.Parent = game.Workspace:WaitForChild(p).Head
r.fg.Text = "Warning 1. Reason: "..a
p.WarningCount = p.WarningCount + 1
wait(5)
r:Destroy()
end)
event run code:
game.ReplicatedStorage.CallWarn:FireServer("MissingBoshjio","res")

5 comments
with remote events/functions your first parameter is always the player instance. So in the server event you should have something like... (player,a,p) as your parameters
it crashes script :P
Can you post the error you are getting?
17:51:26.241 - ServerScriptService.Script:11: attempt to index field 'WarningCount' (a nil value)
Not sure how that error relates to your title but, p is a string("res") and your trying to get something inside of a string(which can not be done.) Could you explain what you are trying to accomplish so we can help you understand?