I have a Mancesa 1.6 gpf toilet and I replaced the flapper but it still has a slight leak is there a replacement seal I can put on the hole that the flapper seals to?

Usually the same place you got the flapper from has replacement seats as well. Is this not the case where you are? Most plumbing supply and hardware stores should have the replacement seats.

Inspect flopper. From my experience, every other new flopper is warped, and leaking. This may be the case. If not, feel the mouth of the flopper seat on the bottom of the overflow tube, making sure there is no chip or broken edge. Yes, you can buy retrofit seal that fits over the seat, but I would strongly suggest you replace entire overflow tube as the retrofit seal is only a temporary solution

Let us know how you did

Milo