Why does it say attempt to index nil with Get Descendants?

The Tower has descendants inside of it but I think it's telling me that it does not have any descendants

for i, object in ipairs(towerToSpawn:GetDescendants()) do
	if object:IsA("BasePart") then
		PhysicsService:SetPartCollisionGroup(object, "Tower")
		
	end
end

``

the tower probably has not loaded yet which is why it is set to nil try to use Instance:WaitForChild() in the line where you set the variable

local towerToSpawn = game:GetService("Workspace"):WaitForChild("Tower")