attempt to get length of a number value?

Edit: Code:

local Seats = workspace.Seats
local seatList = {}
--local seatAvail = {} -- Not needed!

local Players = game:GetService("Players")


-- commented out for me to debug since i don't have these modules!
--local RSModule = game:GetService("ReplicatedStorage").PublicGameModules

--- Modules

--local EventTable = require(RSModule.Events)
--local Sounds = require(RSModule.Popup)

function ResetSeatList()
    print("resetseat activated")
    for i = 1, Seats:GetChildren() do
        print("for loop no." .. i)
        seatList[i] = Seats[i]
    end

end

function StartGame()

    print("Starting!!!...")
    for i, v in pairs(Seats:GetChildren()) do
        seatList[i] = v.Position
    end


    for _, player in ipairs(Players:GetChildren()) do
        if(#seatList > 0) then
            local randSeatAvail = math.random(1, #seatList) -- 3

            local chosenSeat = seatList[randSeatAvail] -- seatAvail[1]

            local character = player.Character
            if not character or not character.Parent then
                character = player.CharacterAdded:wait()
                wait(1) -- needed to wait for the character to actually load before moving it...
            end

            character.HumanoidRootPart.Position = chosenSeat -- seatList[3]
            table.remove(seatList, randSeatAvail)
        end
    end

end
--\/\/\/\/ Hax!! 
Players.PlayerAdded:connect(function(plr)
    StartGame()
end)

--// start game from event #StartGame
--EventTable.StartGame.OnServerEvent:Connect(StartGame)

just uncomment bits you need and remove the Players.PlayerAdded event in this script since you're handling the players through your remote event.

Not great modification but it should help you with coding this type of system. The bit:

    local character = player.Character
    if not character or not character.Parent then
        character = player.CharacterAdded:wait()
        wait(1) -- needed to wait for the character to actually load before moving it...
    end

This is rather slow and hacky, but it works as an example. You would need to hook up each player to their seat position on load.

A much better way to make this work would be something like:

local Seats = workspace.Seats
local seatList = {}
local GamePlayers = {}
local GameEnd = false


local Players = game:GetService("Players")

-- commented out for me to debug since i don't have these modules!
--local RSModule = game:GetService("ReplicatedStorage").PublicGameModules

--- Modules

--local EventTable = require(RSModule.Events)
--local Sounds = require(RSModule.Popup)

function ResetSeatList()
   -- print("resetseat activated")
    for i,Seat in pairs(Seats:GetChildren()) do
        --print("for loop no." .. i)
        if(Seat.Occupant == nil) then
            seatList[#seatList+1] = Seat.Position
        end
    end

end



function GameLoop()

    for player,SeatTble in pairs(GamePlayers) do
        local character = player.Character
        local chosenSeat = SeatTble.mySeat
        if(SeatTble.isSitting == false) then
            SeatTble.isSitting = true
            character.HumanoidRootPart.CFrame = CFrame.new(chosenSeat) -- seatList[3]
        end
    end
    wait()
    if(GameEnd == false) then
        GameLoop()
    end
end


Players.PlayerAdded:connect(function(plr)
    if(#seatList > 0) then
        local randSeatAvail = math.random(1, #seatList) -- 3

        local chosenSeat = seatList[randSeatAvail] -- seatAvail[1]

        local character = plr.Character
        if not character or not character.Parent then
            character = plr.CharacterAdded:wait()
        end
        GamePlayers[plr] = {
            mySeat = chosenSeat,
            isSitting = false
        }

        table.remove(seatList, randSeatAvail)
    end


end)

Players.PlayerRemoving:connect(function(plr)
    if(GamePlayers[plr] ~= nil) then
        GamePlayers[plr] = nil
    end
    ResetSeatList()
end)

The above example will need to keep track on who jumps up away from their seat and re add the seat to the global seatList Table for it to work flawlessly.

Just an idea on how to do this. Might need to change a lot of it to get it to work with your events and modules though.

hope this helps!