ACX Submission is required to be MP3 at a minimum constant bit rate of 192. It is strongly suggested you do it in mono (one blue wave) and not stereo (two blue waves).
When you get your work ready. File > Export > Export as MP3.
If you submit in mono, Channel Mode doesn’t make any difference.
Are you submitting a short sound test? That’s a good idea rather than reading the whole book and only then find out you did something wrong.
https://forum.audacityteam.org/t/how-to-send-a-test-to-acx/49588/1
You should not be doing your work directly in MP3. That can create sound distortion. Export raw readings as WAV (Microsoft) 16-bit. This is the backup if your computer dies in the middle of production. Oh, yes, it does happen.
Then, when you get done with editing, processing, mastering, etc, Export your Edit Master as WAV and only then make the MP3 for them.
If they find problems with your submission, go back to the WAV to fix it and make a whole new MP3.
Koz