Magnitude needs to be fixed?

local player = game.Players.LocalPlayer
local mouse = player:GetMouse()
p = 0

function getDistance()
    return player.Character and player:DistanceFromCharacter(workspace.Door.Root.Position) or math.huge
end


while wait() do
    if getDistance() < 20 then
        p = 1
        scrn = Instance.new("ScreenGui",player.PlayerGui)
        txt = Instance.new("TextLabel",scrn)
        txt.BackgroundTransparency = 1
        txt.Text = "Click to open door."
        txt.Position = UDim2.new(0, 600,0, 500)
        txt.Size = UDim2.new(0, 500, 0, 50)
        txt.TextScaled = true
        txt.TextColor3 = BrickColor.new("White").Color



    end
end

You were not updating the magnitude variable, so you were always checking the distance of the character's spawning location to the door's position. DistanceFromCharacter is also more efficient.