Error Message: Attempt to index local 'leader' (a nil value) won't go away. Help?

That because it can't find the leaderstats.

local plr = player.name
local leader = plr.leaderstats

You put that plr is a string value ( player.Name not player.name )

You can navigate to player with LocalPlayer ( only work with local script )

local leader = game.Players.LocalPlayer:FindFirstChild("leaderstats")

And wait until it can find leaderstats.

repeat wait() until game.Players.LocalPlayer:FindFirstChild("leaderstats")
local leader = game.Players.LocalPlayer.leaderstats

Your script will be like this.

local button = script.Parent

local function onButtonActivated()

local plr = plr.Name
repeat wait() until game.Players.LocalPlayer:FindFirstChild("leaderstats")
local leader = game.Players.LocalPlayer.leaderstats
local price = script.Parent.Parent.Price.Value
local car = workspace.CarsInGame:FindFirstChild(plr)


if leader.Cash.Value >= price then
    leader.Cash.Value = leader.Cash.Value - price
    if car then
        car:Destroy()
        local clone = game.ReplicatedStorage.Cars.ArmyTruck:clone()
        clone.Parent = game.Workspace.CarsInGame
        clone.Name = plr
    else
        local clone = game.ReplicatedStorage.Cars.ArmyTruck:clone()
        clone.Parent = game.Workspace.CarsInGame
        clone.Name = plr 
    end
end

script.Parent.Parent:Destroy()

end                            

button.Activated:Connect(onButtonActivated)