"there is an error at game.Workspace:Terrain"

-- server script (ServerScriptService) 
local remoteEvent = game.ReplicatedStorage:FindFirstChild("CreateProjectileEvent")

if not remoteEvent then
    remoteEvent = Instance.new("RemoteEvent")
    remoteEvent.Name = "CreateProjectileEvent"
    remoteEvent.Parent = game.ReplicatedStorage
end

local function createProjectile(player, mousePosition)
    local part = Instance.new("Part")
    part.Size = Vector3.new(3, 1, 1)
    part.BrickColor = BrickColor.new("Really red")
    part.Material = Enum.Material.Foil
    part.Position = player.Character.HumanoidRootPart.Position + Vector3.new(0, 5, 0)
    part.Velocity = (mousePosition - part.Position).unit * 100
    part.Anchored = false
    part.CanCollide = true
    part.Parent = game.Workspace

local function onPartTouch(other)
        if other:IsDescendantOf(player.Character) then
            return
        end
        local explosion = Instance.new("Explosion")
        explosion.Position = part.Position
        explosion.BlastRadius = 20
        explosion.BlastPressure = 100000
        explosion.DestroyJointRadiusPercent = 0 -- This will make the explosion not delete terrain around it
        explosion.Parent = game.Workspace
        part:Destroy()
    end

part.Touched:Connect(onPartTouch)
end

remoteEvent.OnServerEvent:Connect(createProjectile)

-- local script (StarterGui or StarterPlayerScripts) 
local remoteEvent = game.ReplicatedStorage:WaitForChild("CreateProjectileEvent")
local player = game.Players.LocalPlayer
local mouse = player:GetMouse()

local function onMouseClick()
    remoteEvent:FireServer(mouse.Hit.p)
end

mouse.Button1Down:Connect(onMouseClick)

-- Make sure to insert a RemoteEvent into ReplicatedStorage and name it 'CreateProjectileEvent' if it does not already exist. 
-- The error was due to trying to clear terrain in a game without terrain. The line has been removed.