In order to fix this issue, would you mind opening the 'Activity Monitor' utility on your computer (normally located under Applications > Utilities) to see if there are any other processes associated with Dropbox that may be preventing our desktop application from being removed from your device?
Once you have launched 'Activity Monitor', if you see any items that refer to Dropbox, or recognize any programs that you may have linked to Dropbox. Could you please perform the following steps?
Click on the line that references the process to select it
Click on the 'X' at the top left corner of the screen. Be careful not to click on the 'X' that closes the window, but the one that will 'Force a process to quit'.
Once this is done, please use the normal steps to uninstall the app.