mp3 support was removed in Camtasia 9. Here is a rather large thread about it. It was re-added in Camtasia 2019, but we now use encoding built into the operating system. Unfortunately, Windows 7 does not have the encoding built-in and that's why you don't see the mp3 option. You need Windows 8 or 10 to be able to export to mp3.
I would say it's highly unlikely this is going to change in the future. If you are unable to upgrade your OS or you don't want to use another application for this functionality on Windows 7 you can request a refund.
Sorry that this probably isn't the answer you were hoping for, Glenn