An older thread (https://www.dropboxforum.com/discussions/101001016/why-can-i-only-back-up-specific-folders/612837 ) (replies are turned off) said:
I just started using the back up feature in Dropbox but it seems to only back up 3 folders: DOWNLOADS, DESKTOP & DOCUMENTS. I don't keep important stuff in any of these. How do I select my 2 most important folders? It doesn't seem to let me choose.
So far I have only seen answers confirming that is true, and tough luck - you are stuck with this maddeningly limited functionality.
I just wanted to say that there is a great solution, but it does require a higher learning curve. It is called rclone (https://rclone.org/dropbox/ ) - free and Open Source software designed to integrate as many different cloud storage solutions as possible, and enable you to copy between them, and also back and forth with any folder on your computer that you so choose. Perfect! The downside is the learning curve as by default, you need to type commands into a command line to use it (I've created script files to do that work on a monthly basis, so I don't manually type in any commands any more). BUT there are multiple GUI's (Graphical User Interfaces) for rclone, including one in beta from the rclone team themselves - this provides a 'point and click' interface that is easier to use. I haven't tried any of the GUI's so I can't recommend any, but I'd say it is well worth a shot!