I initially was going to post to ask for help, but I finally got this figured out!!
First of all, Dropbox has a feature to save screenshots automatically. Go to preferences and then Backup and you should find a setting to "share screenshots to dropbox". Now the problem/solution:
So, I have a new Lenovo laptop running windows 10. I noticed that instead of saving screenshots to Dropbox when I press the "print screen" button on my keyboard that it would open Snip & Sketch software. I tried uninstalling that software and now it just opens Snipping Tool instead. For the life of me I couldn't figure out how to simply save a screenshot into Dropbox . I tried restarting, reinstalling. I tried all the different variations of taking a screenshot, such as Windows Key + PrtSc etc etc. I tried removing Snipping Tool with registry edit but couldn't find it. I turned off the setting “Print Screen Shortcut” in the “Easy of Access Keyboard” settings which SHOULD have done the trick but did NOT. It even says I need to restart which I did, and it still didn't work.
Then I read online you can take screenshots with WINDOWS KEY + VOLUME DOWN
And I got the beautiful notification "A SCREENSHOT WAS ADDED TO DROPBOX"
Problem Solved!! For some reason, that command now behaves like the old print screen button used to.
(You might need to also find your “ease of access keyboard settings” and turn off the setting “Print Screen Shortcut” which I did prior to discovering this awkward solution, so if this didn’t work for you, try turning that setting off and trying again)
I hope this helps someone out there. It took me 2 hours to figure this out.