How come the last part of my code doesn't work?

The Player function :GetRankInGroup() is a function of the player object, not the name of the player. On line 12, you were trying to use it on the player's name, not the player themselves.

Thus, it can be changed to this:

local Players = game:GetService("Players")
local platform = Instance.new("Part")
platform.CFrame = CFrame.new(12.5, 1.71, 17.43)
platform.Anchored = true
platform.Transparency = .5
platform.Size = Vector3.new(5, 10, 1)
platform.Parent = workspace.CurrentCamera

function onPlayerAdded(player)
    if  (player:GetRankInGroup(2551575) >= 239) then   
        platform.CanCollide = false
    end
end
Players.PlayerAdded:connect(onPlayerAdded)