Have you tried using 'Custom' to enter the desired parameters? You could try the 'Reset' procedure (File menu), If all else fails, uninstall with a third-party uninstaller/cleaner (Revo) and reinstall.
Gave all three of those suggestions a shot.... to no avail. I can save to mp3 in Music Maker and Reaper... Just interesting that I can't do it in SF. Really like the program... but not much good for creating music for cd's if I can't save in that format.
After selecting and setting the 'Custom' parameters, what happens when click the OK button?
If all else fails, uninstall Sound Forge using a third-party uninstaller and reinstall. I recommend using 'Revo Uninstall', and choosing the 'Advanced' scan option. Revo will uninstall and remove the left-over registry entries and other pertinent data the OS uninstall leaves behind, which can cause issues to reoccur.. Revo Uninstall has pro and freeware versions. The free version is adequate in most cases. https://www.revouninstaller.com/products/revo-uninstaller-free/
Sorry @David-Wolfe, I do not have an answer and I do not recall reading of that particular behavior. I recommend you request a Magix support ticket(Support ticket instructions) Unofficially, you could try emailing Magix at: infoservice@magix.net