The pre-reveal preview image is the assets your customers will see right after minting an NFT from your collection before the collection is revealed.
There are three main things that can be causing your preview image to be uneligible.
File Outside Asset's Folder
Your preview image should be inside the folder where you have your assets to be your NFTs and should be at the same level as your assets. Don't put your preview image inside an additional folder
File Format Not Allowed
The allowed formats for your preview image are:
Videos aren't allowed as valid preview image for your pre-reveal preview.
The preview image should be named 'preview' followed by the file format: