well, starter pack is not inserted into a player when the game starts; instead, a new folder named "Backpack" is made and inserted into the player to hold all the equipable tools;

so lets say i have a tool in starterpack named "gun"

to access it; i would have to do: player.Backpack.gun

Create a StarterGear

You could create a StarterGear if it doesn't exist. Just check if it exists and if it doesn't, make a new one.

local StarterGear = Player:FindFirstChildOfClass('StarterGear')
if (not StarterGear) then
    StarterGear = Instance.new('StarterGear', Player)
end

Create a custom system

More effort, but will solve your problem and can be more reliable (like if you want to put the tools in a certain order).

Just save the player's tools using a dictionary or some other method and clone it when they spawn in.

--saving tools as starter
local savedTools = {}

savedTools[Player] = {Item1, Item2, Item3}

--spawning tools
Players.PlayerAdded:Connect(function(player)
    player.CharacterAdded:Connect(function(character)
        local tools = savedTools[player]
        for i, tool in pairs(tools) do
            --clone in backpack
        end
    end)
end)

script.Parent.MouseButton1Click:connect(function() 
    local RS = game:GetService("ReplicatedStorage") 
    local item = RS:WaitForChild("Firebrand") 
    local price = 25 
    local player = game.Players.LocalPlayer 
    local stats = player:WaitForChild("leaderstats")
    local SG = player:WaitForChild("StarterGear") or player:FindFirstChild("StarterGear")

-- when u do FindFirstChild u can do

--< if SG then
    --code here
     else
    SG = player:WaitForChild("StarterGear")  -- try to find it again

    --other code to start over on the if SG then
--   end >


    if stats.Souls.Value>=price then
        stats.Souls.Value = stats.Souls.Value - price
        local cloned = item:Clone()
        cloned.Parent = player.Backpack
        cloned.Parent = SG
    end
end)