Which piece is missing?

Hey everyone, I recently joined the BMX world and I'm building my first bike. The thing is, investing in new parts is really expensive, so I'm buying them used. I bought a Cult semi-sealed cassette hub, but unbeknownst to me, it was incomplete. So far, I don't know which part is missing in the bearing area, and the bearings keep coming off the axle 😭 and I don't know how to stop them from getting inside the hub.

Please help me figure out which part is missing 😭😭!!!