Might as well use a print statement to verify that PropService.PropHeld works properly. RunService will do its job regardless of whether it’s on the client or server, but there are certain restrictions. For example, RunService.RenderStepped works on the client but not the server. However, RunService.Heartbeat, which also gets fired every frame (but a little later than RenderStepped) should do the trick.
A little extra reading regarding running code each frame: