I think you've been sold the wrong one. Under the oil that looks like a cassette that fits on to a free hub, in which case it would need the cassette remover tool working with the chain whip.
Once the lock ring is loosened and removed the cassette should slide out from the free hub splines.
Complain to your bike shop and ask them to change it.
I always prefer to clean up oily bits of kit like that before working on them as you have a clearer idea of what needs doing.