I had the same problem, and found the answer here: http://shadowpeakstudios.com/2018/10/22/unity-tutorial-animated-tiles/

Basically for it to work you have to download the 2d-extras from here: https://github.com/Unity-Technologies/2d-extras

Steps:

1. Download the 2d-extras
2. Extract the zip, then drag the "2d-extras-master" folder into your "Assets" folder in Unity
3. Create a new tilemap: Right-Click in Hierarchy > 2D-Object > Tilemap
4. Then right click on your "Assets" folder: Create > Tiles > Animated Tile (will finally show up)

I had the same problem, and found the answer here: http://shadowpeakstudios.com/2018/10/22/unity-tutorial-animated-tiles/

Basically for it to work you have to download the 2d-extras from here: https://github.com/Unity-Technologies/2d-extras

Steps:

1. Download the 2d-extras
2. Extract the zip, then drag the "2d-extras-master" folder into your "Assets" folder in Unity
3. Create a new tilemap: Right-Click in Hierarchy > 2D-Object > Tilemap
4. Then right click on your "Assets" folder: Create > Tiles > Animated Tile (will finally show up)