It seems as though you can’t call Run on Server in the level blueprint, you’ll have to do it on an actor that is owned by the client (playercontroller)
Server RPCs can only be called by a client who owns the actor executing the event graph. For example, a PlayerPawn is owned by the client who controls it - only that client can send Server RPCs on that pawn. The pawn also may own weapons, abilities, inventory items, etc (this is game specific). This can be a hard concept to communicate directly in blueprints. It is something we will work to improve as time goes on.
An explanation by staff where I found this answer after failing to get it to work myself on the level blueprint, but getting it to work by calling it in the characterBP:
The answer by vanlacke is correct, but for anyone who would like to keep level logic in the level BP and still run RPCs, this is the way I work around it without creating a set of RPCs for everything.
In my player controller I have a generic passthrough RPC, that can be re-used with a different ID: