Which paint do I use to paint a chair with varnish?

Over varnish I would Prime and paint with oil based products. Will stick better and is most durable. Spray paint is also good. Lightly sand off sheen and wash down with TSP. Have fun.

After primer I can’t us acrylic paint n when done n then revarnish ..?

Lightly sand to remove any gloss. Prime with Kilz primer. Then paint. You can use any paint you want. Seal with three coats of a water based polyurethane.

Here's some ideas

https://www.hometalk.com/diy/paint/furniture/painting-a-whimsical-chair-with-free-pattern-12907717

https://www.hometalk.com/diy/outdoor/garden/sassy-garden-whimsey-chair--19648738