How do I fix this? Workspace.Coin.Script:4: attempt to index nil with 'leaderstats'

you should add something to check if the hit part is a player and you should add debounce instead of disabling the script

local db = true
script.Parent.Touched:Connect(function(hit)
    local player = hit.Parent:FindFirstChild("Humanoid")
    local plr = game.Players:GetPlayerFromCharacter(hit.Parent)
if player then
   if plr then
    if plr.leaderstats.Cash.Value >= 0 and db == true then
    db = false
      wait()
      script.Parent.Transparency = 1
      script.Parent.CanCollide = false
      plr.leaderstats.Cash.Value = plr.leaderstats.Cash.Value + 5
        wait(1)
    db = true
        script.Parent.Transparency = 0
        script.Parent.CanCollide = true

    end
end
end
    end)

2 Issues I see with your script:

1. You aren't checking if you hit a player's character
2. You are disabling the script as some sort of cooldown, using a Debounce would be much better

local Players = game:GetService("Players")
local DB = false

script.Parent.Touched:Connect(function(Hit)
    local plr = Players:GetPlayerFromCharacter(Hit.Parent)
    if plr and DB == false then
        DB = true --Nobody else can trigger this while DB is true

        --Do whatever you gotta do

        wait(3)
        DB = false --waits 3 seconds, then ends the cooldown
    end
end)