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)