why does my handcuffs keep on saying nil in the output?

###This is because Button1Down passes no parameters, so click is nil. Remove that parameter. If you wanted to get the player like this, use Mouse.Target. Also, all Mouse events have been superseded by UserInputService.

local players = game:GetService("Players")
local plr = players.LocalPlayer -- use more local variables.
local char = plr.Character or plr.CharacterAdded:Wait()
local playerMouse = plr:GetMouse()
local UIS = game:GetService("UserInputService")
local tool = script.Parent
local HRP = char.HumanoidRootPart -- use this instead of torso. R6 and R15 have this
local equipped = false

tool.Equipped:Connect(function(mouse)
    equipped = true
end)

UIS.InputBegan:Connect(function(input)
    if input.UserInputType == Enum.UserInputType.MouseButton1 and equipped then
        local target = mouse.Target
        local targetPlr = players:GetPlayerFromCharacter(target.Parent)

        if target and targetPlr ~= plr then -- make sure we can't arrest ourselves!
            local difference = (HRP.CFrame.p - targetChar.HumanoidRootPart.CFrame.p)
            
            if difference.Magnitude <= 21 then
                targetPlr.Character.HumanoidRootPart.CFrame = game.Workspace.prison.CFrame
            end
        end
    end
end)