Hey. Your artwork has 3000 colours. You can check it here. This is most likely a result of using NPA tools and techniques. Most likely when applying smudgy, blurry highlights. But also the texture on frontal bricks is also not very clean. And the frames around the eyes look like made with some random spray.

All those indistinguishable colours you see in the link should be unified. This would make a cool pixel art, but we cannot accept it at this state. Cleaning it would be a challenge, and probably it would be easier to start the rendering from the beginning (erasing everything but the black outline and the brick shapes, and add colour and texture again)

Tried that out, attempted to understand the best I could. Re-uploaded with what I think is what you meant.

The preview image must also be a pixel art. If you need to scale down your piece, use only nearest neighbor option for it. Otherwise you'll get such blurry not PA image. And such cannot be accepted into the gallery.