I came here after Googling for this very feature, for the same reason as @nikunj-bhatt - I often take screenshots (several times a week) and it's irritating to have to hide the Bookmarks Bar then unhide it each time.
Having found from this thread that it's not possible to move the Bookmarks Bar I explored other options and I've found one that might be useful for others: I created a hotkey to toggle the Bookmarks Bar (show or hide it).
To do this click Alt+P to open Opera Settings, then, at the top right, enter "show bookmarks bar" in the Settings search box. It will come up with two hits. The first one under Appearance you can ignore. It just toggles the visibility of the Bookmarks Bar, same as the menu item does. The second is under Shortcuts. You'll need to expand the Configure shortcuts section to see it. Scroll down the list of commands until you see "Show bookmarks bar" highlighted.
Then hover over "Show bookmarks bar" and a button "Type a shortcut" will appear. Type the hotkey combination you want to use and you're done. Opera will alert you if the hotkey combination is already in use so you don't need to worry about overwriting an existing shortcut. I found Ctrl+Shift+K was a free combination I could use.
After this using your new hotkey will toggle the Bookmarks Bar visibility, alternatingly showing and hiding it. Quick and easy for those screenshots.