You’re using FindFirstChild and trying to use it like it’s a table index.
Try this instead
local ObbiesAvailable = Obbies:GetChildren()
local ChosenObby = ObbiesAvailable[math.random(1,#ObbiesAvailable]:Clone()
ChosenObby.Parent = workspace
FindFirstChild expects a string; the name of the object you are trying to get. It will find the first child in the instance with that name. And unless the names of all of the obbies are numbers, your original code with not work properly.