Hi Niveditha,
This error basically means that there already is an object in-memory for the Agile Central query results. There isn't any specific knowledge I could find we have, but I guess it could be if you have another instance of the Excel add-in running (perhaps last time you closed Excel , it didn't really close), perhaps another sheet where you ran a query.
Can you check to see if another Excel is running in your task manager?
If you didn't find that, then I suggest restarting the Excel itself and reconnecting. If that also didn't help then try to reboot your machine.
Let us know if any of that helped.
Thanks,
Sagi