3D image black background

First you must give us more detail as Rick says, anyway check this tutorial to get understand how to export with alpha channel https://www.premiumbeat.com/blog/quick-tip-exporting-with-alpha-channels-in-after-effects/

No system and workflow details, no screenshot with the modified properties of the problem layer revealed (uu), no mention of the file type for the coin = no idea what is going wrong. My first guess is that the coin is an image and it does not have an alpha channel. My second guess, you do not have the Transparency Grid turned on in the Comp Panel and you do not understand the UI.

Give us more details and maybe we can help. If the image is a PNG or a Tiff then selecting it in the Project Panel will tell you if you have an alpha channel or not. Turning on the transparency grid will also show you if you have transparency.

Hi. Was your coin image created with an alpha background? After Effects creates a black background in all new compositions (you can change this colour) - this is NOT an real background, it shows any alpha chanell as black and it will not render. To check if you have an alpha background on your coin click on the checkerboard button at the bottom of your viewing screen (it's next to "Active Camera") - if you have an alpha background a grey checkerboard will show up. Does that fix it?