Sorry you're having this situation. Other posters may be more familiar with Camtasia 2 - my experience has been TSC for Mac V.3 and up.
A few questions:
1) You say that you're exporting from Camtasia 2 and then trying to convert, meaning a two-step process. Does Camtasia 2 allow you to export/share your project as an .mp4? 1a) If not, what file type are you saving/exporting your project to? I ask because you should have no issue using Handbrake, a free utility to convert your output to .mp4.
2) If question 1 doesn't pan out, would you be willing to: A) Save a new (safety) copy of your project B) Download a trial of Camtasia V.2020 - which is available for Mac/Windows. C) Import the safety copy of your project into V.2020. If you're able to do have no issue exporting to .mp4 which I know the Mac version 3, 2018, 2019 and 2020 can all do, from firsthand experience.