What does this mean and how do I fix it?

--Interact Script

local plr = game.Players.LocalPlayer
local mouse = plr:GetMouse()
local deb = false
local gui = plr.PlayerGui.Interact.Background
local dis = 5

while true do wait()
	if mouse.Target then
			if mouse.Target:FindFirstChild("canInteract") then
			local mag = (plr.Character.Torso.Position - mouse.Target.Position)
				if mag<=dis then 
				gui.Visible = true
				else
				gui.Visible = false
				end
			else
			gui.Visible = false
			end
	end
end

Error:Players.Player.PlayerGui.interactScript:13: attempt to compare userdata with number

I'd try working with Region3 on a project like this, but I've never used it before, so I'll go with your method. Anyway, Vector3's have a property called "Magnitude" to get the distance between points.

local plr = game.Players.LocalPlayer
local mouse = plr:GetMouse()
local deb = false
local gui = plr.PlayerGui.Interact.Background
local dis = 5

while wait() do
    if mouse.Target then
        if mouse.Target:FindFirstChild("canInteract") then
            local mag = (plr.Character.Torso.Position - mouse.Hit.p).Magnitude --The distance between your torso and the target
            if mag<=dis then 
            	gui.Visible = true
            else
            	gui.Visible = false
            end
        else
        	gui.Visible = false
        end
    end
end