1. Create some entity
2. Add a user script to that entity
3. Within the user script, add a method that should be Rpc-ed and add an Rpc call somewhere. For example, I wrote this user script for a trigger entity.
4. Add an RplComponent to the trigger entity.
5. Play (with PeerTool) and it will result in this error: RpcError: Calling a RPC from an unregistered item! itemType='script::Game::SCR_BaseTriggerEntity', rpc=''

If you copy the very same script and save it as a completely new entity, it will work just fine, the issue specifically occurs when using Rpc inside of user scripts.

Here is a test prefab with the sphere trigger user script, be sure to rename it so the script runs.