how to make only one person see a part?

Create the part through a local script.

local Part = Instance.new("Part")
Part.Parent = game.Workspace

Simply use local parts! Local parts are parts that are created in a local script. This is how games with story progression allow some players to move past obstacles while others have to complete whatever they need to before they can progress.

-- Local Script 
p = Instance.new("Part", workspace)

Moreover, you could do what zanyder suggested in the comments and make a part on the server and use remote events to control who sees it.

-- function that is called whenever a story event finishes
function StoryEventEnd(player)
    ClearObstacleEvent:FireClient(player)
end

-- client code

ClearObstacleEvent.OnClientEvent:Connect(function()
    workspace["ObstacleName"]:Destroy()
end)

If this answer helped you, please mark it as correct so others may see it when passing by <: