Can anyone help me with the error a nil value?

So i did this: LocalScript:

local target = script.Parent.Parent.target.Value
local sended = false

function send()
	if sended == false then
		local timer = tonumber(script.Parent.Time.Text)
		sended = true
		game.ReplicatedStorage.JailPlayer:FireServer(timer, target)
	end
end



script.Parent.Send.MouseButton1Click:Connect(send)

script.Parent.Parent.target.Changed:Connect(function()
	sended = false
end)

ServerScript in SSS:

game.ReplicatedStorage.JailPlayer.OnServerEvent:Connect(function(timer, target)
	game.Players:FindFirstChild(target).JailTime.Value = timer	
end)

And the player up here is a nil value ;(

First of all, there's no need to get the player using what you're doing.

When you use FireServer(), the player is already passed to the server script.

Example

LocalScript

local event = game:GetService("ReplicatedStorage").RemoteEvent
event:FireServer() 

ServerScript

local event = game:GetService("ReplicatedStorage").RemoteEvent
event.OnServerEvent:Connect(function(player) -- player is already defined
	
end

So to fix your problems, simply change your server script to the following. (LocalScript needs no change)

Server Script

game.ReplicatedStorage.JailPlayer.OnServerEvent:Connect(function(player,timer, target)
    local targetPlayer = game:GetService("Players")[target]
    targetPlayer.JailTime.Value = timer   
end)