Triggering client event

How can I prevent this error: "Server triggered clientside event eventName, but event is not added clientside" with onMarkerHit which triggers a client event?

addEvent(name,true) clientside and then attach this event to a function. Make sure the player has downloaded all the files and scripts, so the event can actually be triggered.

I think you understood me wrong. I scripted everything correctly and it works but sometimes when player hasn't finished downloading the client script, this error appears for him. How can I check if the download of my client script is finished and loaded?

You can make a table of available players on the server, and when the client finishes downloading (when your script starts, onClientResourceStart with resourceRoot as the element) simply trigger server to add the client into the table.