Cannot mark my video with CC SA licence because Youtube Content ID found 3rd party soundtrack which itself is CC SA licensed
I am not a laywer, but my limited understanding of the situation is this.

I see two issues here:


When you upload a video to youtube you need to grant them certain rights. As Youtube is without any doubt a commercial plattform, beyond others you have to grant them the right to commercially use your video. 
Unfortunately, as your soundtrack is CC NC, you didn't have the right to do so yourself. That's why Youtube is correct in complaining about the song.
"SA" means you have to share your work also as CC SA NC. As Youtube doesn't allow you to select that exact license, it's technically impossible for you to comply with that condition. 
(When you select CC-BY in Youtube, an additional license claim anywhere else, e.g. in the video description, would probably not legally binding)


So after all - if you want to upload a video to Youtube, it must contain only material that


has been created or licensed by you
is under CC BY license
is public domain or CC-0