Open finder > desktop , documents , downloads folders can be removed ( just right click on them and remove from side bar ) , perhaps can't be deleted to trash .
Open finder preferences > sidebar section check the box of them they will again appear in finder side bar .
See the article Change preferences in the Finder on Mac - Apple Support
You can open trash of Mac ( just right click on trash icon and click on open ) , tap once on the drop box folder and click on put back .
Second method is click on finder , take cursor on top menu bar > Go > hold option key and click on library > click on public
Again open trash of Mac and drag and drop the dropbox folder in the location of public folder .
By mistake if you delete the drop box folder in trash and empty the trash , a new folder can be created ( right click in the column and create a new folder and name as Drop box ) .
You had a practice of saving drop box folder in the side bar .
Note : don't change default location of drop box , and save in finder side bar .
See some articles Get to know the Finder on your Mac - Apple Support
Organize files in folders on Mac - Apple Support
