Trying to rip the 2018 Xbox Avatars before they become unusable

[saratogaroad]

You can try AssetStudio instead. Point it at the folder with your assets using File > Load Folder, then go into the Asset List tab and filter for Texture2D. If the textures you want are in the files you have, they'll pop up in there. If they're not, that's also why they're not popping up in AssetRipper and you'll need to find some other way of getting them.