Audio alignment: "can not move clip to frame x"

[DogFilm]

What does this message mean?
I set an audio reference, select "Align audio" and after a while I see that
message in the status bar:
"can not move clip to frame x"
Why can the clip not be moved?
What can the user do when that message appears?
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.kde.org/pipermail/kdenlive/attachments/20190922/a5ee3945/attachment-0001.html>