There's a code example for calling this endpoint using curl in the documentation, and you can also generate some code for calling it using the API v2 Explorer.
If something isn't working as expected, please share the code you're using and the unexpected error or output.
The error is indicating that you're trying to parse some data as JSON, but the data isn't in a valid JSON format, so the parsing is failing.
I don't see anything that mentions Dropbox in the error screenshot, nor do I see anywhere that the stack trace in the error that references the code you posted, so I can't offer much insight. You'll need to debug your app more to track down where/why this is occuring.