I have a 2000 Triumph TT 600. It has a leak at the rubber grommet right above the stator cover. I have taken it to the dealer, but they have been unable to fix the problem using silicone sealant. What is causing this and how can I fix it?

I don't know why but Triumph does not list the grommet as an individually replacable part which is annoying!
But there is no reason why they/should not be able to seal this with a silicone based product.
If it has been attempted & failed, the reason is undoubtedly that the oil was not cleaned sufficiently from the cover and/or grommet before applying.
So you should thoroughly degrease these before attempting to retry.

Good Luck!