An item shows "Not Accepted" when SkinSwap is not able to take it at the moment. There is no fixed list. Common reasons:
The item's price is inflated.
SkinSwap already holds too many of that item.
Other reasons related to the item or the market.
Support cannot change whether an item is accepted, and cannot change or negotiate prices. You can still use your other skins or your balance to trade.
Related articles: How to trade skins, How to sell your skins