how to make a Stun Tool using KeyBind?

You're going to want to be using a local script within your tool to setup the client side. You should be doing something like this (I also added comments to explain some things you did wrong):

local Players = game:GetService("Players") -- You used game.Players, exploiters can rename the services locally and error your scripts. This is just more reliable.
local Player = Players.LocalPlayer -- No need to be attempting to get the player with spamming .Parent, LocalPlayer points to your Player.
local Character = Player.Character or Player.CharacterAdded:Wait() -- This is a bit more advanced syntax, you are more or less checking to see if the Character exists, and if it doesn't simply wait for it to exist.
local StunCooldown = false
local IsStunOn = false


game:GetService("UserInputService").InputBegan:Connect(function(Input, GameProcessed) -- This is capturing when a key is pressed
    if (GameProcessed) then return end -- This is to make sure they aren't trying to say something in chat
    if (Input.KeyCode == Enum.KeyCode.P) then -- This is checking to make sure the key they pressed was P
        if (IsStunOn) then -- This is checking to see if the stun effect is on
            IsStunOn = false -- If it's on, turn it off
        else -- If its not on:
            if (StunCooldown == false) then -- Make sure they aren't in the middle of a cooldown
                StunCooldown = true -- Turn on their cooldown
                IsStunOn = true -- Turn on the IsStunOn bool check
                game:GetService("ReplicatedStorage").RemoteEvent:FireServer() -- Fire a remove event without any arguments for security reasons
                wait(1) -- Wait a second to get rid of the effect and cooldown
                IsStunOn = false -- Turn off the effect
                StunCooldown = false -- Turn off the cooldown
            end 
        end
    end
end)

On the server, you should be doing something like this:

game:GetService("ReplicatedStorage").RemoteEvent.OnServerEvent:Connect(function(Player) -- Listen for the event to be called, the Player who fired the remote is automatically the first argument
    local Character = Player.Character or Player.CharacterAdded:Wait() -- Get the players character or wait for it to exist
    for _, OtherPlayer in pairs(game:GetService("Players"):GetPlayers()) do -- Get every player in the server
        coroutine.resume(coroutine.create(function() -- Create a coroutine / simulate a new OS thread, Lua doesn't actually use more than one OS thread though.
            if (OtherPlayer == Player) then return end -- Make sure it's not the player who fired the remote
            local OtherPlayerCharacter = OtherPlayer.Character -- Get their character
            local OtherPlayerHumanoidRootPart = OtherPlayerCharacter.HumanoidRootPart -- Get their humanoid root part
            if (OtherPlayerHumanoidRootPart.Position - Character.HumanoidRootPart.Position).Magnitude < 10 then -- Make sure they're within 10 studs
                OtherPlayerCharacter.Humanoid.WalkSpeed = 1 -- Set their WS to 1
                OtherPlayerCharacter.Humanoid.JumpPower = 0 -- Set their JP to 0
                wait(0.5) -- Yeild for half a second
                OtherPlayerCharacter.Humanoid.WalkSpeed = 16 -- Set their WS to normal
                OtherPlayerCharacter.Humanoid.JumpPower = 50 -- Set their JP to normal
            end         
        end))
    end
end)

Keep in mind, this is inherently unsecure because every player has network ownership of their Character, meaning all physics is handled by them and replicated to the server. They can change their WalkSpeed on their own and this system can overall be exploited. You should also make checks separately to determine if they are exploiting their WalkSpeed.

Hope this helps.