No, you can't. The wood needs to be clear of any stains or varnish which will prevent the whitewash from soaking into the wood. If you want to paint the table a solid colour with chalk paint, there is no need to remove the varnish first unless it's chipping off. Chalk paint will cover varnish a treat. You may need to paint up to three coats depending on how dark the furniture is and how opaque you'd like the finish to be.