What's the trick attaching copper nut to plastic thread of wc inlet pipe? Thanks

That plastic thread is likely the bottom side of your fill valve. Can you post a picture of your copper pipe? Most of the time we use a flexible chrome plated brass line made specifically for connecting the toilet, your threads may not be a match. The supply tubes I use have there own plastic nut attached.Connecting The Water Supply Line To A New Toilet - Bathroom Plumbing

I'll bet what's happened is that you changed out your old metal ballcock for a plastic one and you're using the old metal nut on the supply over again.
The trick is to be very certain that the nut squares up with the threads when starting. If you start the nut off crossthreaded the metal nut will act as a set of dyes and start to cut new threads. You now have a crossthread started. Don't tighten up a crossthread, it will leak. Try to restart the nut correctly. Good luck, Tom