First of all you need to remember that replication ONLY happens from server to client.
The ball I assume you have set to “Replicates” and “Replicate movement”, then you have to apply the force server-side and the “Replicate movement” takes care of replicating the movement to the clients.
In order for a client to send a RPC to the server (“Run On Server” Event) the client needs to own the actor it is sending from. It could be the PlayerController or Pawn.
The problem was as you said that I made my blueprint in the ball and not in the pawn, I had to make some modifications to adapt it to the pawn but it is now perfectly working.
Thank you very much for your help !