So I’m making a co-op horror game, and it seems to work pretty well so far, but for some reason the other players can’t pick up items, only the host. How do I make it so other players can interact with items, doors, etc… PS: I know it has something to do with that whole replication system, but I have no clue how it works.
Yep. It has to do with RPC specifically. The clients need to tell the server that they want to pick something up. We do that kind of thing with a function that is set to “Run on Server” (check function details panel). It is called on the client and will execute on the server (a message over the network makes this happen).
Ah I see. Is there a video out there that shows me how to do this? I was looking for some tutorial vids but couldn’t find one.
One more thing: the host can also call the function. Since you don’t want to handle a special case for clients vs host, the host can pretend it’s a client and call the same functions. UE4 will just call it locally without sending a message over the network.
And yeah, it gets a lot more complicated then “that whole replication system”. 
There aren’t many end-to-end multiplayer tutorials but there are plenty of tutorials available to get your feet wet. Search YouTube for “UE4 replication” and you’ll find at least a handful that can get you started, including one from Epic. Once you learn more you can search for specific aspects of the system. GL!