Hi Somaya,
It’s been for more than one month that I struggle with this problem. In particular, I couldn’t drag and drop a Timeline into another timeline.
I was getting exactly the same dialog as you, and no matter what folder I selected, I kept still getting access denied.
Today I selected the main Drive (e.g. “Macintosh HD” in my case), and it solved the problem.
Hope this helps,
Jonatan