I finished a song in 2017 BIAB saved it as an .SGU file, then went back to export it as a .mp4. I got this error message:

Access violation at address 00908097 in module 'bbw.exe'. Read of address 000000A0


I don't use BIAB all that often but I think I save these .SGU files in a folder named "BIAB Tracks" and then I go back to save them or export in WAV form. The folder then shows the song in both Band-In-A-Box Song format and WAVE Audio File format.

How do I fix this and am I doing something wrong?

This is normal behaviour. When you create a wav, either by saving or exporting, BIAB will put it in the same folder as the parent SGU/MGU file.

Also, when you next open the BIAB file, that wav (or mp3) will automatically be loaded in the Audio track.

Out of curiosity, did you try exporting as mp3 rather than mp4 to see if that works?