Attempt to index nil with ''UserId"?

I've been creating a Auto-Rank Application Center using this tutorial: https://devforum.roblox.com/t/how-to-create-a-rank-management-system-using-glitch/442725

I have been able to get the GUI working, which if a player gets >= 9 for their score, it fires the server, which I have set up a remote event in replicated storage which fires when the Event is fired. Everything is working, but when the event is run, I get the error "ServerScriptService.Main:9: attempt to index nil with 'UserId'"

Scripts Below

Main Script

game.ReplicatedStorage.SetRank.OnServerEvent:Connect(function()
	local GroupId = 6151289
	local HttpService = game:GetService("HttpService")
	local Server = require(script.Server)
	local domain = "removed for privacy reasons"
	local key = "removed for privacy reasons"
	local player = game.Players.LocalPlayer
	
	if player.UserId == player.UserId then
		Server.SetRank(GroupId, player.UserId, 329)
	end
end)

Server Script

local Server = {}

local HttpService = game:GetService("HttpService")

local Configs = require(script.Parent.Configs)
 

local function Request(Function, RequestBody)
	
	--Before sending the request, add our auth_key to the body
	RequestBody["auth_key"] = Configs.AUTH_KEY
	
	local response = HttpService:RequestAsync(
		{
			-- The website to send the request to. Function is the extended part of the URL for specific functions.
			-- In this case, Function = 'GroupShout'
			-- Example: 
			--	"Configs.BaseUrl..Function" would be equal to: http://test-app.glitch.me/GroupShout
					
			Url = Configs.BaseUrl..Function, 

			-- The request method (all of ours will be POST)
			Method = "POST",

			-- We are sending JSON data in the body
			Headers = {
				["Content-Type"] = "application/json"
			},
			-- The body of the request containing the parameters for the request
			Body = HttpService:JSONEncode(RequestBody)
		}
	)
 
	if response.Success then
		print("Status code:", response.StatusCode, response.Body)
		print("Response body:\n", response.Body)
		
		return response.Body
	else
		print("The request failed:", response.StatusCode, response.Body)
		return response.Body
	end
end

Server.Promote = function(GroupId, UserId)
	assert(typeof(GroupId) == "number", "Error: GroupId must be an integer") -- Throw error if GroupId is not an integer
	assert(typeof(UserId) == "number", "Error: UserId must be an integer") -- Throw error if UserId is not an integer

	local Body = {
		Group = GroupId;
		Target = UserId;
	}
	
	 -- pcall the function 'Request', with arguments 'Promote' and Body
	local Success, Result = pcall(function()
	    return Request('Promote', Body)
	end)
	
	print(Result)
end

Server.Demote = function(GroupId, UserId)
	assert(typeof(GroupId) == "number", "Error: GroupId must be an integer") -- Throw error if GroupId is not an integer
	assert(typeof(UserId) == "number", "Error: UserId must be an integer") -- Throw error if UserId is not an integer
	
	local Body = {
		Group = GroupId;
		Target = UserId;
	}
	
	local Success, Result = pcall(function()
	    return Request('Demote', Body)
	end)
	
	print(Result)
end

Server.SetRank = function(GroupId, UserId, RankId)
	assert(typeof(GroupId) == "number", "Error: GroupId must be an integer") -- Throw error if GroupId is not an integer
	assert(typeof(UserId) == "number", "Error: UserId must be an integer") -- Throw error if UserId is not an integer
	assert(typeof(RankId) == "number", "Error: RankId must be an integer") -- Throw error if RankId is not an integer

	local Body = {
		Group = GroupId;
		Target = UserId;
		Rank = RankId;
	}
	
	local Success, Result = pcall(function()
	    return Request('SetRank', Body)
	end)
	
	print(Result)
end

Server.HandleJoinRequest = function(GroupId, PlayerUsername, Boolean)
	assert(typeof(GroupId) == "number", "Error: GroupId must be an integer") -- Throw error if GroupId is not an integer
	assert(typeof(PlayerUsername) == "string", "Error: PlayerUsername must be a string") -- Throw error if PlayerUsername is not a string
	assert(typeof(Boolean) == "boolean", "Error: Boolean must be a boolean value") -- Throw error if Boolean is not a boolean value

	local Body = {
		Group = GroupId;
		Username = PlayerUsername;
		Accept = Boolean; -- true or false
	}
	
	local Success, Result = pcall(function()
	    return Request('HandleJoinRequest', Body)
	end)
	
	print(Result)
end

Server.GroupShout = function(GroupId, ShoutMessage)
	assert(typeof(GroupId) == "number", "Error: GroupId must be an integer") -- Throw error if GroupId is not an integer
	assert(typeof(ShoutMessage) == "string", "Error: ShoutMessage must be a string") -- Throw error if ShoutMessage is not a string

	local Body = {
		Group = GroupId;
		Message = ShoutMessage;
	}
	
	local Success, Result = pcall(function()
	    return Request('GroupShout', Body)
	end)
	
	print(Result)
end

return Server

Hello, tylerisyummy! The problem is that LocalPlayer returns nil in a regular/server script as the server doesn't run on clients. Instead, use the automatic "player" parameter in the OnServerEvent event. Change your main script to this:

game.ReplicatedStorage.SetRank.OnServerEvent:Connect(function(player)
    local GroupId = 6151289
    local HttpService = game:GetService("HttpService")
    local Server = require(script.Server)
    local domain = "removed for privacy reasons"
    local key = "removed for privacy reasons"

    if player.UserId == player.UserId then
        Server.SetRank(GroupId, player.UserId, 329)
    end
end)

Please accept this answer and upvote it if it helped you.

The reason maybe that you didn't concatenate two strings together like so do Make sure it's in a server script.

    if player == "player" ..player.UserId then
        Server.SetRank(GroupId, player.UserId, 329)
    end
end)