Hi there,
I've seen a few questions like this already and from what I've seen; 1.20 assets are currently not supported by Mine-Imator.
As another person has said in specific, "Mine-Imator simply doesn't "understand" 1.20 worlds."
To further elaborate, there's a dropdown in Mine-Imator that allows you to choose the game version of Minecraft that you'd like the application to operate from. As of now, said dropdown only goes as far as game version 1.19.3.
People have experienced all types of bugs when trying to import their worlds from game version 1.20 regardless. Bugs have ranged as far as their worlds going transparent, to simply not loading at all.
However, as a long-winded alternative. You can try and make 1.20 models (trees, items, etc.) in Modelbench and import them from there until Mine-Imator has official support for version 1.20.
Hope this helps!