From what I know, when you upload a game to Roblox it becomes a Roblox property, meaning that they can use your game for any purprose, included economical ones.
So it’s impossible to copyright a game on Roblox, since the game itself is a Roblox property and not your property (I might be wrong).
If you want to have a better understanding of your rights and the Roblox rights on your work the following link has everything you need to know: