I have the game, but I'm afraid I don't have an answer for you. I haven't purchased the berserker booster pack, and I've no idea what it could mean by sandbox environment. I assume you have a dojo already, right? It should give each brute in your dojo 100 fights. If you haven't already made a dojo, you need to make one and add brutes to it. If you've already got brutes in your dojo, it sounds like a bug and you should contact support.