Can't add a new emitter to Niagara system

I was wondering if anyone could help me out in figuring out why I’m unable to add a new emitter to the Niagara system I’m working on. When I right-click & select Add Emitter, I get the following message:

No results, check your filter

I have no idea what the heck this message is referring to. I just created this Niagara system, I haven’t done anything with the filter, whatever its talking about.

I don’t know what to do, thus why I need help.