Hi! Check out this post I made over on Doomworld that shows the Compositor layout, and ensure you've got "Viewer Node" placed and connected to your "Render Layers" output. Once that's connected, you should be getting an image output when you click "Render" in Palette Studio.
https://www.doomworld.com/forum/topic/119920-palette-studio-v12-create-doom-spri...
Oh! Also if you're using the whole palette range (like 0-255 for a 256 color palette) you can leave that range box blank and it'll use the whole range.