The information in the thread referenced in the previous post is mostly regarding RealBand, and it's incorrect for Band-in-a-Box.

Starting with Band-in-a-Box 2018, we use better methods of compressing audio. We use Microsoft's Media Foundation framework which is available on newer versions of Windows (starting with Vista) and replaces the older Audio Compression Manager. Depending on the compression format, you need a newer version of Windows, otherwise we fall back to the older methods of conversion.

For MP3's, Media Foundation requires Windows 8 or higher. Otherwise you'll need to have an MP3 encoder codec installed, and it must be an ACM (Audio Compression Manager) codec.

If you are running Band-in-a-Box 2020 64-bit on Windows 10, you should be able to render MP3's and you shouldn't need to install any additional software. Perhaps there is an issue with a specific format. I suggest that you go to the Audio menu | Export Song as Audio File. Write down or screenshot the settings you have in this dialog, and select MP3 under Format. Report back what you find.

Andrew
PG Music Inc.