Guide to configure Dropbox API keys with WordPress Backup plugin.
Visit Dropbox Apps page and log in to create API keys and add them to corresponding settings of this page.
If you don’t have any apps, press Create Apps button from Dropbox App Console. Select Dropbox API option, then choose the access type, Sandbox App or Full Dropbox App. Name your app, then hit Create App button. You will be redirected to a dashboard page, where the App key and App secret will be provided.
Note, that Full Dropbox App and Sandbox App have different API keys. Make sure to add yours to the correct options of Backup WD plugin.