It's probably because you don't have the plug-in that provides the method installed on your copy of InDesign Server. I'm pretty sure that it's part of the debug install--it doesn't (or shouldn't) exist in the released desktop version.
Thanks for your reply, but I didn't get much out of it.
Actually I made one plugin which is supposed to be run on both desktop Indesign and Indesign Server. I wrote a script through which I was trying to invoke server plugin when the script was run through Indesign server command prompt. But actually the script invoked desktop plugin and not server plugin.
count = app.qatest("CountPlugins");
which was giving an error.
I would be very thankful if you can throw some light on this issue in more explainatory language.