You’re using a Server script (I assume), and a Server Script doesn’t know what “LocalPlayer” is.
Make a Script in ServerScriptService and read more down below.
Here’s how to fix your code.
local Players = game:GetService("Players") -- Get the Players Service
local MarketplaceService = game:GetService("MarketplaceService") -- Get the MarketplaceService Service
local Workspace = game:GetService("Workspace") -- Get the Workspace Service
local GamepassID = 0000000 -- Replace the 0000000 with your Gamepass ID
Players.PlayerAdded:Connect(function(Player) -- Waiting for the player to join and giving it a variable called "Player".
Player.CharacterAdded:Connect(function(Character) -- Waiting for the " Player "'s Character.
if MarketplaceService:UserOwnsGamePassAsync(Player.UserId, GamepassID) then -- We're using the MarketplaceService Service to check whether the Player owns the gamepass, by using their UserId, as gamepass purchases are always stored on a Player's USERID. You don't have to add a " == true ", using this will already check if you own it, if yes, you can continue code, if not, you don't have to do anything.
-- Player owns the Gamepass:
Workspace:FindFirstChild("HelicopterDoor"):Destroy() -- You don't have to make it invisible and cancollide off, "Destroying" the part will do the exact same.
end -- End of function "MarketplaceService"
end) -- End of function "Character"
end) -- End of function "Player"
If you were to destroy something with a server script, it will replicate across the entire server. Basically meaning if someone with the gamepass joins, everyone will be able to go into the area with the gamepass door. Also, you don’t need to get workspace with game:GetService('Workspace') you can just use workspace.