How do I make the trail work in a live server?

local lPlayer = game.Players.LocalPlayer
local char = lPlayer.Character
local debounce = false

function onKeyPress(inputObject, gameProcessedEvent)
	if inputObject.KeyCode == Enum.KeyCode.F then
		if not debounce then
			debounce = true
			local clonetrail = script.Parent.trail:Clone()
			char.Humanoid.WalkSpeed = 64
			clonetrail.Parent = char
			wait(5)
			clonetrail:Destroy()
			char.Humanoid.WalkSpeed = 16
			wait(8)
			debounce = false
		end
	end
end

game:GetService("UserInputService").InputBegan:connect(onKeyPress)

You need to use the Attachment0 and Attachment1 properties of the Trail object. Your script also errors on line 2, since the Character needs time to load.

local plr = game:GetService("Players").LocalPlayer
local char = plr.Character or plr.CharacterAdded:Wait()

function maketrail()
    if not char:FindFirstChildOfClass"Trail" then
        local trail = Instance.new"Trail"
        trail.Parent = char
        trail.Attachment0 = waitforchild(char.Head, "TrailAttachment0")
        trail.Attachment1 = waitforchild(char.HumanoidRootPart, "TrailAttachment1")
    end

    if not char:FindFirstChild('TrailAttachment0', true) then
        local attachment0 = Instance.new"Attachment"
        attachment0.Name = "TrailAttachment0"
        attachment0.Parent = char:WaitForChild"Head"
    end
    
    if not char:FindFirstChild('TrailAttachment1', true) then
        local attachment1 = Instance.new"Attachment"
        attachment1.Name = "TrailAttachment1"
        attachment1.Parent = char:WaitForChild"HumanoidRootPart"
    end
end

function waitforchild(parent, child)
    parent:WaitForChild(child)
end

game:GetService("ContextActionService"):BindAction(
    "MakeTrail",
    maketrail,
    false,
    Enum.KeyCode.F
)