I have the same problem.
On Catalina 10.15.3 I can edit and save mp3 files with Wavepad 9.51.
When I upgrade to Wavepad 10.16 I can't save any edited mp3 file.
Installed Wavepad 10.16 is downloaded from NCH site, and previously I have uninstalled old version 9.51.
My computer is a Mac mini late 2018 with security chip T2.
JMGM