Hello there @johnnyvav! Let’s see what we can do here.
From the UE 5.5 community guidelines, here’s an extract on how to render the alpha channel:
Both .png and .exr image formats offer the ability to output the alpha channel with the image, but first you will need to enable this support. From the editor menu bar, select Edit > Project Settings > Engine > Rendering > Postprocessing > Enable alpha channel support in post processing (experimental) > Linear color space only.
Once this change is applied, you will be prompted to restart the engine. To create a transparent background, you will need to hide opaque objects in your scene such as Sky and Atmospheric Fog.
The full guide can be found here: