How do you detect if the left mouse button is being held down?

mouse = game:GetService("Players").LocalPlayer:GetMouse()

mouse.Button1Down:Connect(function()
-- skript here
end)

for up use

mouse = game:GetService("Players").LocalPlayer:GetMouse()

mouse.Button1Up:Connect(function()
-- skript here
end)

Remember it needs to be local script I hope this helps.

Utilize the Button*Down/Up methods on the Mouse, obtained by calling :GetMouse() on the LocalPlayer object.

local mouse = game.Players.LocalPlayer:GetMouse();

With this function, we can check if the Button1Down is fired.

local Mouse = game:GetService('Players').LocalPlayer:GetMouse();
local BeingHeldDown = false;
Mouse.Button1Down:Connect(function(DownFunction)
    BeingHeldDown = true;
end)

Mouse.Button1Up:Connect(function(UpFunction)
    BeingHeldDown = false;
end);

If we use a variable to remember the state of the mouse, this will allow us to check if the player is holding down the left mouse button. This will allow us to do cool loops like this:

while BeingHeldDown do
    wait()
    print('Mouse is being held down!')
end

or something like this:

game:GetService('UserInputService').InputBegan:Connect(function(Key, Process)
    Key = Key.KeyCode -- I don't need anything else that Key provides

    if (Key == Enum.KeyCode.E) and BeingHeldDown then
        print('The E button is being pressed while the MouseButton1 is being held.')
    end
end)

Using variables to log event states can prove to be very useful in many cases.