Hi, do you call this directly when a client joins the game? If so consider making this a replicated or repnotify variable instead of using a RPC to set it.
Else maybe when you spawn it on the server, then the actor has not replicated to the client and does not exist on the client when you try to sent a reference to it (don’t know just a wild guess). If so, then either try using a delay to give the client time to catch up, or again try using repnotify instead of a RPC.