Why do i keep getting the attempt to index nil with 'Character' with my script?

Im trying to make an attack on titan game and I want to make the titan shifter power first, I scripted it so when they press F they change into a titan, I already scripted it so the camera would stick to the titan but the body wont move so i made it so the character changes into the titan but i am getting an error that im trying to index nil with Character which i said like 'local char = plr.Character' I dont know why im getting an error, I clearly described plr as game.Players.LocalPlayer so why am i getting this error! am i not suppose to do this script in a server script and a local script? I tried that already in a local script i cant use OnServerEvent. BTW This is both the scripts right here :

Server Script :

--Variables
local event = game.ReplicatedStorage.Remotes.BeginTitanShift
local plr = game.Players.LocalPlayer
local char = plr.Character
local rig = game.Workspace.PlayerShifterTitan

--Code


event.OnServerEvent:Connect(function(shift)
	local camera = workspace.CurrentCamera
	camera.CameraType = "Follow"
	camera.CameraSubject = rig.HumanoidRootPart
	script.Parent.RemoteEvent:FireServer()
        char = rig
end)

Local Script(In StarterCharacterScripts) :

--Variables
local key = Enum.KeyCode.F
local uis = game:GetService("UserInputService")
local event = game.ReplicatedStorage.Remotes.BeginTitanShift
local player = game.Players.LocalPlayer




--Code

uis.InputBegan:Connect(function(input, g)
	if g then return end
	if input.UserInputType == Enum.UserInputType.Keyboard then
		if input.KeyCode == key or Enum.KeyCode.F then
			event:FireServer()
		end
	end
end)

If you know the error tell me plzzzzz

Try this

--Variables
local event = game.ReplicatedStorage.Remotes.BeginTitanShift
local plr = game.Players.LocalPlayer
repeat wait() until plr.Character 
local char = plr.Character
local rig = game.Workspace.PlayerShifterTitan

--Code


event.OnServerEvent:Connect(function(shift)
    local camera = workspace.CurrentCamera
    camera.CameraType = "Follow"
    camera.CameraSubject = rig.HumanoidRootPart
    script.Parent.RemoteEvent:FireServer()
        char.PrimaryPart = rig.PrimaryPart ----Changed it---
end)