Yes you can. A flat acrylic paint will stick just fine. But, you might want to wait a little while. You applied two coats of paint and this paint hasn't cured yet. The true sheen of a paint isn't realized until it cures. This can take from 2 weeks to 1 month.
Wait for 2 weeks and see if it is toning down or the sheen is lower than when it was first applied. If it is still too shiny, go ahead and repaint with a flat.