You cannot convert .ses files (sessions) from Adobe Audition to FLAC, or any other audio format. The sessions contains just metadata for Audition project and not the actual audio data. If you want to convert the audio, you would need to do that for the data references in the project.
Convert Adobe Audition session data to Audio files encoded by Flac - free lossless audio codec.
We have found 2 software records in our database eligible for .ses to .flac file format conversion.
Microsoft Windows software - convert ses to flac on Windows
Professional audio editing software from Adobe
Convert from ses file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned ses to flac. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Apple macOS / Mac OS X software - convert ses to flac on OS X
Mac version of Adobe's prime audio editing software