Problem with Mixdown; impossibile to export

The default is to mix down everything in your session. The alternative is to highlight everything you need to mix down, and select Multitrack>Mix down session to a new file>Time selection. You will get a similar choice if you go to Export>Multitrack mixdown. This would give you additional choices as to exactly what happens to your mixed file, rather than it just being opened immediately in Waveform view.