Why can't I cast value to Object?

I'm working on a quest system that I want to be as modular as possible. (I've somewhat succeeded in that) I've run into an error when I want to complete the quest. The end result is to get the client to change the text of a TextLabel upon completing a quest.

I

What I'm trying to do is FireClient inside of a ServerScript, but it's not letting me. I know there are other ways to get this done, but I kinda want to use RemoteObjects (just for the halibut)

Here's my code. The main problems are on:

Lines 48 on Server-side and Lines 60-62 on Client-side

The end result is to get the client to change the text of a TextLabel when a quest is completed.

Server-side code:

event.OnServerEvent:connect(function( ... )
	local tuple = { ... }
	local player = tuple[1]
	local pStats = RPGLib.getPlayerStats(player)
	local questID = pStats["QST"].Value
	local questIDs = {"testquest"}
	
	if tuple[4] == "Accept" then
		print("made it to accept")
		if questID == tuple[2] then
			if questID == questIDs[1] and tuple[3] == "Mobs" then
				print("made it to quest1")
				local questInfo = pStats["QINF"]
				
				local type = questInfo:FindFirstChild("Type")
				type.Value = "Mobs"
				
				local mobsObjective = questInfo:FindFirstChild("MobsObjective")
				mobsObjective.Value = 5
				
				local mobsCurrent = questInfo:FindFirstChild("MobsCurrent")			
				
				
				local mobsCompleted = questInfo:FindFirstChild("MobsCompleted")
				local activeQuest = pStats["AQST"]
				activeQuest.Value = true
				while activeQuest.Value == true and mobsCompleted.Value == false do
					wait()
					if mobsCurrent.Value == mobsObjective.Value then
						mobsCompleted.Value = true
						break
					else
						return
					end
				end		
			end
		end
	elseif tuple[4] == "Complete" then
		print("made it to quest complete attempt")
		if questID == tuple[2] then
			local player = tuple[1]
			if questID == questIDs[1] and tuple[3] == "Mobs" then
				print("made it to 'complete quest1'")
				local questInfo = pStats["QINF"]
				local QuestConfig = {1, 1, player} --Order of values: Difficulty, Level
				RPGLib.giveQuestReward(player, QuestConfig)

				event:FireClient("Finished testquest", player)
				
				local type = questInfo:FindFirstChild("Type")
				type.Value = " "
				
				local mobsObjective = questInfo:FindFirstChild("MobsObjective")
				mobsObjective.Value = 0
				
				local mobsCurrent = questInfo:FindFirstChild("MobsCurrent")			
				mobsCurrent.Value = 0
				
				local mobsCompleted = questInfo:FindFirstChild("MobsCompleted")
				local activeQuest = pStats["AQST"]
				activeQuest.Value = false
				
				local newQuestId = pStats["QST"]
				newQuestId.Value = " "
			end
		end
	end
end)

Client-side code:

local player = game.Players.LocalPlayer
local replicatedstorage = game:GetService("ReplicatedStorage")
local event = replicatedstorage:WaitForChild("RemoteEvent")
local mouse = player:GetMouse()

local gui = script.Parent
local chatprompt = gui:WaitForChild("ChatPrompt")
local questwindow = gui:WaitForChild("QuestWindow")

local NPCpic = questwindow:WaitForChild("NPCpic")
local NPCname = questwindow:WaitForChild("NPCname")
local QuestDesc = questwindow:WaitForChild("QuestDesc")
local questId = questwindow:WaitForChild("QuestID")
local questType = questwindow:WaitForChild("QuestType")

local accept = questwindow:WaitForChild("Accept")

local activeQuestWindow = false
local activeChatPrompt = false





event.OnClientEvent:connect(function( ... )
	local tuple = { ... }
	if tuple[1] == "PromptChat" then
		if tuple[8] == player.Name then
			chatprompt.Visible = true
			activeChatPrompt = true
		end
		if tuple[2] == "Quest" then
			mouse.KeyDown:connect(function(key)
				local name = tuple[3]
				local picid = tuple[4]
				local desc = tuple[5]
				local questid = tuple[6]
				local questtype = tuple[7]
				if activeChatPrompt == true and string.lower(key) == "e" then
					NPCname.Text = name
					NPCpic.Image = "rbxassetid://" .. tostring(picid)
					QuestDesc.Text = desc
					questId.Value = questid
					questType.Value = questtype
		
					questwindow.Visible = true
					
			
				end
			end)
			accept.MouseButton1Click:connect(function()
				local desc = tuple[9]
				QuestDesc.Text = desc
			end)
		end
		
	elseif tuple[1] == "CloseChatPrompt" then
		chatprompt.Visible = false
		activeChatPrompt = false
	elseif tuple[1] == "Finished testquest" and player.Name == tuple[2] then
		local desc = tuple[10]
		QuestDesc.Text = desc
	end
end)