I see that you are new to the form.
Welcome.
The people who volunteer on this forum providing answers and solutions to working in ArchiCAD deserve the best description of a problem that you can provide to facilitate their answer.
In this case, there are many things that can be wrong - your source file, your transparency in rendering, the rendering engine, etc.
I need more information in order to help you.
You should have a photo editor to change file types in your application arsenal, or:
Make another rendering, save it as a jpg and post it so we can get closer to your solution.
Dwight Atkinson