Just bought this Samsung galaxy Ao3s (66k) at slot yesterday and find out yesterday night during video call it doesn't have front camera flash has to use touch to make video calls which is frustrating for me, I want to return it tomorrow at slot lekki where I bought it, will they accept it and give me another phone around that prize?

Don't bother returning, they will not accept it. That means you didn't do your homework very well before purchasing the phone. You didn't check the specification before buying, just sell n get another one.
You have already tored the carton n activate the phone for use. If you are offered open box phone, can you buy it? You can go n try your luck sha, don't let me discourage you..

I seriously doubt that except it has a problem for which they would accept it back since its on warranty abi guaranty

I don't think Samsung makes phone with front cam flash, most times the light from the screen is good enough for decent night time video calls.

The box has been opened It is pretty difficult to get replacement around that price. As someone said you didn't do your research before purchase.