Ok, I want to thank you for your time for reading my post. I need advice on how to remove this old candelier, I have already unscrew the two nails but the cover plate is still stuck to the ceiling, I have also tried truning it anticlockwise but no luck .
It could be a number of things keeping it on. Most likely it is paint as stated in another answer. It could also be that there is some sort of pressure fit here between the cover plate and the base of the fixture, or some sticky substance other than paint. No matter what it is of these, most likely some prying from the ceiling with force should get it off, just be careful if you want to keep the ceiling nice.
There's four things that could be holding it in place to the bracket attached to the ceiling.
1 The threaded nipple that should be protruding through the center hole that the ring is screwed onto to support the weight of the chandelier. Doesn't look like there's one present.
2 The two screws that hold the cover to the bracket. You have apparently taken those out.
3 Paint that can stick the cover to the bracket or the ceiling itself. If you've turned it, that has likely broken up any paint present.
4 These covers can sometimes have a rolled-over edge or indents that grip onto the edges of the bracket. If that's the case, you need to pry it away with ideally a small pry bar, bearing on something like a putty knife to prevent indenting into the ceiling.