Create bookmark button disappeared from the Bookmark Panel?!

Hello,

I use Adobe Acrobat Pro DC and try to create some bookmarks for a pdf file, but I can't find create bookmark button in the Bookmark Panel like in the previous versions! What happened to it and why it's not there? I use it many times, but today; when I needed to create some bookmarks, it does not exist.

I took a screenshot of my Bookmark Panel. Why I can't see and find the button for creating bookmarks?

Best,

Masoud