I need a part that cancollide = true if it's under 50 speed, and cancollide = false if it's over?

It looks like you may be mixing some server and client components of scripting. If the Part is the parent of the script then it would likely be a server Script; however, the LocalPlayer can only be used from a client LocalScript.

I would suggest to make a LocalScript and put it into StarterPlayerScripts or StarterGui. Next I'll make a few modifications to your code to put in the LocalScript.

local P = game.Workspace.Part --change this to where the Part is located if this is not it
local Player = game.Players.LocalPlayer
local Torso = (Player.Character or Player.CharacterAdded:wait()):WaitForChild("Torso") -- waits until the character first spawns, yes it's a little complicated

Player.CharacterAdded:connect(function(Character) -- gets the new Torso if the character respawns
	Torso = Character:WaitForChild("Torso")
end)

while true do
	wait(0.05)
	if Torso.Velocity.magnitude >= 50 then -- an if else statement is better to use than two if statements in this case
		P.CanCollide = false
		P.Transparency = 0.5
	else
		P.CanCollide = true
		P.Transparency = 0
	end
end