Error in saving adobe audition multitrack file

You're in Multitrack view, so Ctrl+S would perform a save, but not of your file - it would save the session. I can't really tell very much about this from your screengrab, but there are some things you need to be aware of: Firstly, that Audition doesn't edit MP3 files at all - it decodes them and opens them as wav files, and that's what you end up editing - essentially a copy of your original. Secondly, there's no way you are going to be able to save a single file like that anyway - it needs to be mixed down first before you can either save it as a file, or Export it. If you try the Export route, you'll be invited to save a mixdown, and that will result in a final file.

But then we're back to the MP3 issue again. You started with MP3 files, which are compressed and inherently lower quality. To save your result as an MP3, your files have to be re-encoded, so re-compressed, and the quality will inevitably be lower. A lot of people do this, and don't seem to care, but I thought that I ought to point this out in case you weren't aware.

What I can't tell is why the session refused to save, although there has been a file issue with the latest release. The cure for this at present is to download the beta version from the CC app, which fixes the problem. Whether that's related I can't tell, but it's almost certainly worth downloading that and trying again, bearing in mind what I've just written.