My script doesnt detect when a player is holding a tool and then clicks a block?

First, create a local script within the Tool:

local Player = game:GetService("Players").LocalPlayer
local Mouse = Player:GetMouse()

local ReplicatedStorage = game:GetService("ReplicatedStorage")
local RemoteEvent = ReplicatedStorage:WaitForChild("RemoteEventToolTest")
local Tool = script.Parent

Tool_Forage.Activated:Connect(function()
    local Target = Mouse.Target
    if Target then
        if Target:FindFirstChild("ClickDetector") then
        RemoteEvent:FireServer(Target)
        end
    end
end)

Now, what this does is the Tool will detect if it is activated (clicked). It will then detect the Target your mouse is clicking. If the Target has a 'ClickDetector' within it, then it will fire the RemoteEvent.

Now, create a server script that contains this:

local ReplicatedStorage = game:GetService("ReplicatedStorage")
local RemoteEvent = ReplicatedStorage:WaitForChild("RemoteEventToolTest")

local function ToolActivated (Target)
    local Target_Workspace = game.Workspace:FindFirstChild(Target)
        if Target_Workspace then
            Target_Workspace.Transparency = 0.5
        else
            print("Target is not existing in the Workspace.")
        end
end

RemoteEvent.OnServerEvent:Connect(ToolActivated)

This server script will fire the function 'ToolActivated' whenever the local script within the tool fires the RemoteEvent. The tool then passes the parameter 'Target'. This Target then gets passed on to the server script. Now, the server script will find the 'Target' on the Workspace, and sets its Transparency to the indicated value. If the Target cannot be found, it will print out 'Target is not existing in the Workspace.' Note that since that a 'server-sided script' sets the Transparency, all the Players will see it.